Displays with direct-lit backlight units and light spreading structures

ABSTRACT

A display may have a pixel array such as a liquid crystal pixel array. The pixel array may be illuminated with backlight illumination from a backlight unit. The backlight unit may include a printed circuit board, a plurality of light-emitting diodes mounted on the printed circuit board, at least one light spreading layer formed over the printed circuit board that spreads light received from the plurality of light-emitting diodes, a partially reflective layer formed over the at least one light spreading layer, a color conversion layer formed over the partially reflective layer, a collimating layer formed over the color conversion layer, a brightness enhancement film formed over the collimating layer, and a diffuser formed over the brightness enhancement film. The at least one light spreading layer may include two light spreading layers with elongated protrusions that are rotated relative to each other.

This application claims the benefit of provisional patent applicationNo. 62/642,539, filed Mar. 13, 2018, which is hereby incorporated byreference herein in its entirety.

BACKGROUND

This relates generally to displays, and, more particularly, to backlitdisplays.

Electronic devices often include displays. For example, computers andcellular telephones are sometimes provided with backlit liquid crystaldisplays. Edge-lit backlight units have light-emitting diodes that emitlight into an edge surface of a light guide plate. The light guide platethen distributes the emitted light laterally across the display to serveas backlight illumination.

Direct-lit backlight units have arrays of light-emitting diodes thatemit light vertically through the display. If care is not taken,however, a direct-lit backlight may be bulky or may produce non-uniformbacklight illumination.

SUMMARY

A display may have a pixel array such as a liquid crystal pixel array.The pixel array may be illuminated with backlight illumination from abacklight unit. The backlight unit may include an array oflight-emitting diodes and a light reflector that helps reflect lightfrom the light-emitting diodes through the pixel array. Eachlight-emitting diode may be placed in a respective cell.

The backlight unit may include a printed circuit board, a plurality oflight-emitting diodes mounted on the printed circuit board, at least onelight spreading layer formed over the printed circuit board that spreadslight received from the plurality of light-emitting diodes, a partiallyreflective layer formed over the at least one light spreading layer, acolor conversion layer formed over the partially reflective layer, acollimating layer formed over the color conversion layer, a brightnessenhancement film formed over the collimating layer, and a diffuserformed over the brightness enhancement film.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an illustrative electronic device having adisplay in accordance with an embodiment.

FIG. 2 is a cross-sectional side view of an illustrative display inaccordance with an embodiment.

FIG. 3 is a top view of an illustrative light-emitting diode array for adirect-lit backlight unit in accordance with an embodiment.

FIG. 4 is a cross-sectional side view of an illustrative light-emittingdiode array with a thermally conductive layer adhered to a printedcircuit board with light-emitting diodes in accordance with anembodiment.

FIG. 5 is a cross-sectional side view of an illustrative light-emittingdiode array with a radiative cooling coating on a printed circuit boardwith light-emitting diodes in accordance with an embodiment.

FIG. 6 is a top view of an illustrative light-emitting diode array for adirect-lit backlight unit having four rectangular light-emitting diodesin each cell in accordance with an embodiment.

FIG. 7 is a top view of an illustrative light-emitting diode array for adirect-lit backlight unit having light-emitting diodes with differentproperties in each cell in accordance with an embodiment.

FIG. 8 is a cross-sectional side view of an illustrative light-emittingdiode array having a light redirecting layer between light-emittingdiodes in accordance with an embodiment.

FIGS. 9 and 10 are cross-sectional side views of illustrativelight-emitting diodes with and without a reflector layer such asdistributed Bragg-reflector in accordance with an embodiment.

FIG. 11 is a cross-sectional side view of an illustrative light-emittingdiode array having a reflective layer in accordance with an embodiment.

FIG. 12 is a cross-sectional side view of an illustrative light-emittingdiode array having encapsulant with a planar upper surface overlight-emitting diodes in accordance with an embodiment.

FIG. 13 is a cross-sectional side view of an illustrative light-emittingdiode array having encapsulant with a curved upper surface over eachlight-emitting diode in accordance with an embodiment.

FIG. 14 is a cross-sectional side view of an illustrative light-emittingdiode array having encapsulant with a curved upper surface and arecessed portion over each light-emitting diode in accordance with anembodiment.

FIG. 15 is a cross-sectional side view of an illustrative light-emittingdiode array having encapsulant with dopants evenly distributedthroughout the encapsulant material in accordance with an embodiment.

FIG. 16 is a cross-sectional side view of an illustrative light-emittingdiode array having encapsulant with dopants at the upper surface of theencapsulant material in accordance with an embodiment.

FIG. 17 is a cross-sectional side view of an illustrative light-emittingdiode array having encapsulant with dopants at the lower surface of theencapsulant material in accordance with an embodiment.

FIG. 18 is a cross-sectional side view of an illustrative dopant withportions having different densities in accordance with an embodiment.

FIG. 19 is a cross-sectional side view of an illustrative dopant havinga shape to control orientation of the dopant in encapsulant material inaccordance with an embodiment.

FIG. 20 is a cross-sectional side view of an illustrative backlight unithaving a patterned layer formed on the encapsulant material inaccordance with an embodiment.

FIG. 21 is a cross-sectional side view of an illustrative light-emittingdiode array having encapsulant with a textured upper surface overlight-emitting diodes in accordance with an embodiment.

FIG. 22 is a cross-sectional side view showing a method for heatingsolder in a backlight unit in accordance with an embodiment.

FIG. 23 is a cross-sectional side view of an illustrative backlight unithaving a light spreading layer coupled to the encapsulant in accordancewith an embodiment.

FIG. 24 is a cross-sectional side view of an illustrative backlight unithaving a first light spreading layer below a second light spreadinglayer in accordance with an embodiment.

FIG. 25 is a cross-sectional side view of an illustrative backlight unithaving a first light spreading layer below a second light spreadinglayer with an intervening wave guide layer in accordance with anembodiment.

FIG. 26 is a cross-sectional side view of an illustrative backlight unithaving encapsulant with respective portions having a parabolic uppersurface over each light-emitting diode in accordance with an embodiment.

FIG. 27 is a cross-sectional side view of an illustrative backlight unithaving light leakage promotion structures between each light-emittingdiode in accordance with an embodiment.

FIG. 28 is a cross-sectional side view of an illustrative display havinga light spreading layer with microlenses on the upper surface and thelower surface of the light spreading layer in accordance with anembodiment.

FIG. 29 is a cross-sectional side view of an illustrative display havingtwo light spreading layers that are rotated relative to each other inaccordance with an embodiment.

FIG. 30 is a cross-sectional side view of an illustrative lightspreading layer having protrusions in accordance with an embodiment.

FIG. 31 is a cross-sectional side view of an illustrative lightspreading layer having recesses in accordance with an embodiment.

FIG. 32 is a top view of an illustrative light spreading layer havingprotrusions or recesses in accordance with an embodiment.

FIGS. 33 and 34 are top views of illustrative light spreading layershaving protrusions formed by partial-cube structures in accordance withan embodiment.

FIG. 35 is a perspective view of an illustrative protrusion for a lightspreading layer that is in the shape of a tapered pyramid in accordancewith an embodiment.

FIG. 36 is a cross-sectional side view of an illustrative backlight unithaving microlenses on a lower surface of a partial reflective layer andan encapsulant with a planar upper surface over the light-emittingdiodes in accordance with an embodiment.

FIG. 37 is a cross-sectional side view of an illustrative backlight unithaving microlenses on a lower surface of a partial reflective layer andencapsulant portions with planar upper surfaces over each light-emittingdiode in accordance with an embodiment.

FIG. 38 is a cross-sectional side view of an illustrative backlight unithaving microlenses on a lower surface of a partial reflective layer andencapsulant portions with curved upper surfaces over each light-emittingdiode in accordance with an embodiment.

FIG. 39 is a cross-sectional side view of an illustrative backlight unithaving microlenses on a substrate that is separated from thelight-emitting diodes by an air-gap in accordance with an embodiment.

FIG. 40 is a cross-sectional side view of an illustrative separatelyformed backlight enhancement film and collimating layer in accordancewith an embodiment.

FIG. 41 is a cross-sectional side view of an illustrative optical filmthat serves as both a backlight enhancement film and a collimating layerin accordance with an embodiment.

FIG. 42 is a cross-sectional side view of an illustrative display havingedge coatings in accordance with an embodiment.

FIG. 43 is a diagram showing how the positions of light-emitting diodesmay be dithered to improve performance of the display in accordance withan embodiment.

FIG. 44 is a top view of an illustrative display having light-emittingdiodes arranged in positions that are dithered in accordance with anembodiment.

FIG. 45 is a top view of an illustrative display where eachlight-emitting diode is covered by encapsulant that is offset relativeto the light-emitting diode in accordance with an embodiment.

FIG. 46 is a cross-sectional side view of an illustrative displayshowing how a slab of encapsulant with a tuned thickness may be formedover light-emitting diodes in accordance with an embodiment.

FIG. 47 is a cross-sectional side view of an illustrative displayshowing how support structures may be included to preserve thestructural integrity of a slab of encapsulant in accordance with anembodiment.

FIG. 48 is a cross-sectional side view of an illustrative display havingsupport structures that both preserve the structural integrity of a slabof encapsulant and serve as light-leakage promotion structures inaccordance with an embodiment.

FIG. 49 is a cross-sectional side view of an illustrative colorconversion layer having a phosphor layer in accordance with anembodiment.

FIGS. 50A-50C are cross-sectional side views of an illustrative colorconversion layer having a phosphor layer with reflective structures inaccordance with an embodiment.

FIGS. 51A and 51B are top views of an illustrative phosphor layer withreflective structures in accordance with an embodiment.

FIG. 52 is a cross-sectional side view of an illustrative colorconversion layer having a phosphor layer with Rayleigh scatteringdopants in accordance with an embodiment.

FIG. 53 is a cross-sectional side view of an illustrative colorconversion layer having an optical film with prisms over a phosphorlayer in accordance with an embodiment.

FIG. 54 is a cross-sectional side view of an illustrative colorconversion layer having a phosphor layer that is patterned to reduce thepath length of off-axis light in accordance with an embodiment.

FIG. 55 is a cross-sectional side view of an illustrative colorconversion layer having an optical film with prisms over a phosphorlayer that is patterned to reduce the path length of off-axis light inaccordance with an embodiment.

FIG. 56 is a diagram showing illustrative steps for attaching apre-soldered light-emitting diode to a printed circuit board inaccordance with an embodiment.

FIG. 57 is a cross-sectional side view of an illustrative backlight thatincludes an optical film with a low-index layer laminated between acollimating layer and brightness-enhancement film in accordance with anembodiment.

FIG. 58 is a cross-sectional side view of an illustrative backlight witha light spreading layer that has concave light spreading features on anupper surface and a lower surface in accordance with an embodiment.

FIG. 59 is a cross-sectional side view of an illustrative backlight witha light spreading layer that has concave light spreading features on alower surface and convex light spreading features on an upper surface inaccordance with an embodiment.

FIG. 60 is a cross-sectional side view of an illustrative backlight witha light spreading layer that has first and second layers havingelongated protrusions that are rotated relative to each other inaccordance with an embodiment.

FIG. 61 is a cross-sectional side view of an illustrative backlight witha color conversion layer that includes prisms on an upper surface inaccordance with an embodiment.

FIG. 62 is a cross-sectional side view of an illustrative backlight withlight-emitting diodes covered by a dome of encapsulant in accordancewith an embodiment.

FIG. 63 is a cross-sectional side view of an illustrative backlight withlight-emitting diodes covered by a dome of encapsulant and interposedbetween reflective structures in accordance with an embodiment.

FIG. 64 is a cross-sectional side view of an illustrative backlight withlight-emitting diodes covered by a dome of encapsulant and interposedbetween reflective structures having curved upper surfaces in accordancewith an embodiment.

FIG. 65 is a cross-sectional side view of an illustrative backlight withlight-emitting diodes interposed between reflective structures andcovered by a slab of encapsulant in accordance with an embodiment.

FIG. 66 is a cross-sectional side view of an illustrative backlight withlight-emitting diodes covered by a slap of encapsulant in accordancewith an embodiment.

FIG. 67 is a cross-sectional side view of an illustrative backlight withlight-emitting diodes covered by a slab of encapsulant and reflectivematerial on the encapsulant in accordance with an embodiment.

FIG. 68 is a cross-sectional side view of an illustrative backlight withlight-emitting diodes that interposed between reflective structures andthat are covered by a slab of encapsulant and reflective material on theencapsulant in accordance with an embodiment.

FIG. 69 is a cross-sectional side view of an illustrative backlight withlight-emitting diodes surrounded by specular reflective material inaccordance with an embodiment.

FIG. 70 is a cross-sectional side view of an illustrative backlight inwhich a partially reflective layer is omitted and a light spreadinglayer with protrusions on an upper surface and a lower surface isincluded in accordance with an embodiment.

FIG. 71 is a cross-sectional side view of an illustrative backlight inwhich a partially reflective layer is omitted and a light spreadinglayer includes first and second layers that are laminated together inaccordance with an embodiment.

FIG. 72 is a cross-sectional side view of an illustrative backlight inwhich a color conversion layer includes reflective structures inaccordance with an embodiment.

FIG. 73 is a cross-sectional side view of an illustrative backlight withlight-emitting diodes that are dithered in accordance with anembodiment.

DETAILED DESCRIPTION

Electronic devices may be provided with backlit displays. The backlitdisplays may include liquid crystal pixel arrays or other displaystructures that are backlit by light from a direct-lit backlight unit. Aperspective view of an illustrative electronic device of the type thatmay be provided with a display having a direct-lit backlight unit isshown in FIG. 1. Electronic device 10 of FIG. 1 may be a computingdevice such as a laptop computer, a computer monitor containing anembedded computer, a tablet computer, a cellular telephone, a mediaplayer, or other handheld or portable electronic device, a smallerdevice such as a wrist-watch device, a pendant device, a headphone orearpiece device, a device embedded in eyeglasses or other equipment wornon a user's head, or other wearable or miniature device, a television, acomputer display that does not contain an embedded computer, a gamingdevice, a navigation device, an embedded system such as a system inwhich electronic equipment with a display is mounted in a kiosk orautomobile, equipment that implements the functionality of two or moreof these devices, or other electronic equipment.

As shown in FIG. 1, device 10 may have a display such as display 14.Display 14 may be mounted in housing 12. Housing 12, which may sometimesbe referred to as an enclosure or case, may be formed of plastic, glass,ceramics, fiber composites, metal (e.g., stainless steel, aluminum,etc.), other suitable materials, or a combination of any two or more ofthese materials. Housing 12 may be formed using a unibody configurationin which some or all of housing 12 is machined or molded as a singlestructure or may be formed using multiple structures (e.g., an internalframe structure, one or more structures that form exterior housingsurfaces, etc.).

Housing 12 may have a stand, may have multiple parts (e.g., housingportions that move relative to each other to form a laptop computer orother device with movable parts), may have the shape of a cellulartelephone or tablet computer, and/or may have other suitableconfigurations. The arrangement for housing 12 that is shown in FIG. 1is illustrative.

Display 14 may be a touch screen display that incorporates a layer ofconductive capacitive touch sensor electrodes or other touch sensorcomponents (e.g., resistive touch sensor components, acoustic touchsensor components, force-based touch sensor components, light-basedtouch sensor components, etc.) or may be a display that is nottouch-sensitive. Capacitive touch screen electrodes may be formed froman array of indium tin oxide pads or other transparent conductivestructures.

Display 14 may include an array of pixels 16 formed from liquid crystaldisplay (LCD) components or may have an array of pixels based on otherdisplay technologies. A cross-sectional side view of display 14 is shownin FIG. 2.

As shown in FIG. 2, display 14 may include a pixel array such as pixelarray 24. Pixel array 24 may include an array of pixels such as pixels16 of FIG. 1 (e.g., an array of pixels having rows and columns of pixels16). Pixel array 24 may be formed from a liquid crystal display module(sometimes referred to as a liquid crystal display or liquid crystallayers) or other suitable pixel array structures. A liquid crystaldisplay for forming pixel array 24 may, as an example, include upper andlower polarizers, a color filter layer and a thin-film transistor layerinterposed between the upper and lower polarizers, and a layer of liquidcrystal material interposed between the color filter layer and thethin-film transistor layer. Liquid crystal display structures of othertypes may be used in forming pixel array 24, if desired.

During operation of display 14, images may be displayed on pixel array24. Backlight unit 42 (which may sometimes be referred to as abacklight, backlight layers, backlight structures, a backlight module, abacklight system, etc.) may be used in producing backlight illumination45 that passes through pixel array 24. This illuminates any images onpixel array 24 for viewing by a viewer such as viewer 20 who is viewingdisplay 14 in direction 22.

Backlight unit 42 may include a plurality of optical films 26 formedover light-emitting diode array 36. Light-emitting diode array 36 maycontain a two-dimensional array of light sources such as light-emittingdiodes 38 that produce backlight illumination 45. Light-emitting diodes38 may, as an example, be arranged in rows and columns and may lie inthe X-Y plane of FIG. 2. Light-emitting diodes 38 may be mounted onprinted circuit board 50 (sometimes referred to as substrate 50) and maybe encapsulated by encapsulant 52 (sometimes referred to as transparentencapsulant 52).

Light-emitting diodes 38 may be controlled in unison by controlcircuitry in device 10 or may be individually controlled (e.g., toimplement a local dimming scheme that helps improve the dynamic range ofimages displayed on pixel array 24). The light produced by eachlight-emitting diode 38 may travel upwardly along dimension Z throughoptical films 26 before passing through pixel array 24.

Optical films 26 may include films such as light spreading layer 28,partially reflective layer 30, color conversion layer 34 (which mayinclude phosphor layer 40 and partially reflective layer 41),collimating layer 44, brightness enhancement film 46, diffuser layer 48,and/or other optical films.

Light-emitting diodes 38 may emit light of any suitable color (e.g.,blue, red, green, white, etc.). With one illustrative configurationdescribed herein, light-emitting diodes 38 emit blue light. To helpprovide uniform backlight across backlight unit 42, light fromlight-emitting diodes 38 may be spread by light spreading layer 28.After passing through light spreading layer 28, light fromlight-emitting diodes 38 may pass through partially reflective layer 30.Partially reflective layer 30 (sometimes referred to as dichroic layer30 or dichroic filter layer 30) may be configured to reflect some lightfrom the LEDs and transmit some light from the LEDs. Partiallyreflective layer may include a multi-Bragg reflector and a diffuserlayer in one possible embodiment. Light that is reflected off ofpartially reflective layer 30 may be recycled (e.g., the reflected lightwill reflect off of other layers such as substrate 50 before reachingpartially reflective layer 30 again). Light that is transmitted throughpartially reflective layer 30 then passes through color conversion layer34 (which may sometimes be referred to as a photoluminescent layer).

The transmission of partially reflective layer 30 may be selected tomaximize the efficiency of display 14. Lowering the transmission of bluelight (e.g., from the light-emitting diodes) through the partiallyreflective layer increases the amount of blue light that is recycled.However, recycling more light may cause more light to be absorbed byprinted circuit board 50 (or other layers below partially reflectivelayer 30). Increasing the transmission of blue light may cause morevisible artifacts, however. Therefore, the transmission of the partiallyreflective layer may be selected to optimize efficiency and uniformityof the display. The reflectance of printed circuit board 50 mayinfluence the optimum transmission level of partially reflective layer30. In one illustrative embodiment, printed circuit board 50 may have areflectance of about 90% and partially reflective layer 30 may reflect50% of blue light from light-emitting diodes 38. Increasing thereflectance of printed circuit board 50 would increase the optimumreflectance of partially reflective layer 30.

Color conversion layer 34 may convert the light from LEDs 38 from afirst color to a different color. For example, when the LEDs emit bluelight, color conversion layer 34 may include a phosphor layer 40 (e.g.,a layer of white phosphor material or other photoluminescent material)that converts blue light into white light. If desired, otherphotoluminescent materials may be used to convert blue light to light ofdifferent colors (e.g., red light, green light, white light, etc.). Forexample, one layer 34 may have a phosphor layer 40 that includes quantumdots that convert blue light into red and green light (e.g., to producewhite backlight illumination that includes, red, green, and bluecomponents, etc.). Configurations in which light-emitting diodes 38 emitwhite light (e.g., so that layer 34 may be omitted, if desired) may alsobe used. In addition to phosphor layer 40, color conversion layer 34 mayinclude a partially reflective layer 41. Partially reflective layer 41(sometimes referred to as a dichroic layer or dichroic filter layer) mayreflect all red and green light and partially reflect blue light, forexample.

If desired, color conversion layer 34 and partially reflective layer 30may be formed as a single integral layer. This may reduce the thicknessof the optical film stack-up.

By the time light from light-emitting diodes 38 reaches collimatinglayer 44, the light has been converted from blue to white and has beenhomogenized (e.g., by the light spreading layer). Collimating layer 44(sometimes referred to as microlens layer 44 or microlens array diffuser44) may collimate off-axis light. One or more brightness enhancementfilms 46 may be included to further help collimate light 45 and therebyincrease the brightness of display 14 for user 20. Finally, backlightunit 42 may include diffuser layer 48 to homogenize light from the arrayof light-emitting diodes.

FIG. 3 is a top view of an illustrative light-emitting diode array forbacklight 42. As shown in FIG. 3, light-emitting diode array 36 maycontain row and columns of light-emitting diodes 38. Each light-emittingdiode 38 may be associated with a respective cell (tile area) 38C. Thelength D of the edges of cells 38C may be 2 mm, 18 mm, 1-10 mm, 1-4 mm,10-30 mm, more than 5 mm, more than 10 mm, more than 15 mm, more than 20mm, less than 25 mm, less than 20 mm, less than 15 mm, less than 10 mm,less than 1 mm, less than 0.1 mm, greater than 0.01 mm, greater than 0.1mm, or any other desired size. If desired, hexagonally tiled arrays andarrays with light-emitting diodes 38 that are organized in othersuitable array patterns may be used. In arrays with rectangular cells,each cell may have sides of equal length (e.g., each cell may have asquare outline in which four equal-length cell edges surround arespective light-emitting diode) or each cell may have sides ofdifferent lengths (e.g., a non-square rectangular shape). Theconfiguration of FIG. 3 in which light-emitting diode array 36 has rowsand columns of square light-emitting diode regions such as cells 38C ismerely illustrative.

If desired, each cell 38C may have a light source that is formed form anarray of light-emitting diode dies (e.g., multiple individuallight-emitting diodes 38 arranged in an array such as a 2×2 cluster oflight-emitting diodes at the center of each cell 38C). For example,light source 38′ in the leftmost and lowermost cell 38C of FIG. 3 hasbeen formed from a 2×2 array of light-emitting diodes 38 (e.g., fourseparate light-emitting diode dies). In general, each cell 38C mayinclude a light source 38′ with a single light-emitting diode 38, a pairof light-emitting diodes 38, 2-10 light-emitting diodes 38, at least twolight-emitting diodes 38, at least 4 light-emitting diodes 38, at leasteight light-emitting diodes 38, fewer than five light-emitting diodes38, or other suitable number of light-emitting diodes. Illustrativeconfigurations in which each cell 38C has a single light-emitting diode38 may sometimes be described herein as an example. This is, however,merely illustrative. Each cell 38C may have a light source 38 with anysuitable number of one or more light-emitting diodes 38. The diodes 38in light-emitting diode array 36 may be mounted on a printed circuitboard substrate that extends across array 36 or may be mounted in array36 using other suitable arrangements.

Light-emitting diodes 38 in light-emitting diode array 36 may generateheat. If care is not taken, the resulting heat gradient (e.g., withareas of the array closer to the light-emitting diodes being hotter thanareas of the array between the light-emitting diodes) may cause a heatgradient in pixel array 24. The color of light emitted by pixel array 24may be dependent on temperature. Therefore, a high thermal gradient inthe pixel array can negatively affect display performance.

FIGS. 4 and 5 are cross-sectional side views of illustrativelight-emitting diode arrays with layers that help distribute heat fromlight-emitting diodes 38 (and prevent a thermal gradient in pixel array24). As shown in FIG. 4, light-emitting diodes 38 may be attached toconductive pads 54 (e.g., solder pads) of printed circuit board 50 usingconductive material (e.g., solder) 56. A thermally conductive layer 60may be attached to the bottom surface of printed circuit board 50 usingadhesive layer 58. Adhesive layer 58 may conduct heat in the negativeZ-direction (e.g., from printed circuit board 50) to thermallyconductive layer 60. Thermally conductive layer 60 may then distributeheat within the XY-plane.

Thermally conductive layer 60 may be formed from any desired material.For example, thermally conductive layer 60 may be formed from aluminum,graphite, a carbon-fiber reinforced sheet, carbon nanotubes, or metalparticles. Thermally conductive layer 60 may have a thermal conductivityof greater than 200 W/mK, greater than 300 W/mK, greater than 400 W/mK,between 200 W/mK and 400 W/mK, or another desired thermal conductivity.Thermally conductive layer 60 may have a thickness 62 of less than 0.1millimeters, less than 0.2 millimeters, about 55 microns, greater than20 microns, or another desired thickness.

Adhesive layer 58 may be a pressure sensitive adhesive layer. Ifdesired, adhesive layer 58 may include additive 64 (e.g., metalparticles) to increase thermal conductivity of the adhesive layer.

In another embodiment, shown in FIG. 5, a radiative cooling coating maybe attached to printed circuit board 50. As shown in FIG. 5, layer 66(sometimes referred to as a coating layer, radiative cooling layer orradiative cooling coating) may be attached to a bottom surface of theprinted circuit board 50 (e.g., using adhesive 58). Layer 66 may includemetal particles that are embedded in a polymer sheet. The metalparticles may, for example, emit infrared light, thereby cooling theadjacent layers.

As shown in FIGS. 4 and 5, light-emitting diodes 38 are attached tosolder pads on printed circuit board 50 using solder. If the overlaparea between the solder and the light-emitting diodes is low, thestrength of the bond between the solder and the light-emitting didoesmay be low. To increase the overlap area between the solder and thelight-emitting diodes (and accordingly, the bonding strength and SMTyield), the light-emitting diodes may have a rectangular design, asshown in FIG. 6.

FIG. 6 is a top view of illustrative light-emitting diodes having arectangular (non-square) shape. As shown, each light-emitting diode inlight-emitting diode array 36 may have a width (W) and a length (L) thatis longer than the width (e.g., greater than the width, at least twiceas long as the width, at least 1.5 times as long as the width, at least3 times as long as the width, less than 3 times long than the width,etc.). In other words, each light-emitting diode may have a non-squarerectangular shape. Each cell (tile area) 38C may have an associated fourlight-emitting diodes 38-1, 38-2, 38-3, and 38-4. As shown in FIG. 6,light-emitting diodes 38-1, 38-2, 38-3, and 38-4 are arranged in a 2×2grid within cell 38C. Light-emitting diode 38-1 is in the upper-left ofthe grid, light-emitting diode 38-2 is in the upper-right of the grid,light-emitting diode 38-3 is in the lower-left of the grid, andlight-emitting diode 38-3 is in the lower-right of the grid.Light-emitting diodes 38-1 and 38-4 may have lengths that are parallelto the Y-axis, whereas light-emitting diodes 38-2 and 38-3 may havelengths that are parallel to the X-axis (and rotated 90° relative to thelengths of 38-1 and 38-4). With this arrangement, each light-emittingdiode may have a non-square rectangular outline (for increased bondingstrength) while ensuring uniform backlight (because the fourlight-emitting diodes in each tile will together emit uniformbacklight). The light-emitting diodes in each tile may be controlledtogether. Each light-emitting diode in a given cell may have any desiredpositioning and orientation. Each light-emitting diode in a given cellmay have any desired pattern of solder. Each light-emitting diode in agiven cell may have any desired encapsulant additives of any desiredparticle size and distribution.

Another arrangement for light-emitting diodes in a light-emitting diodearray 36 is shown in FIG. 7. As shown in FIG. 7, each tile 38C may havean associated plurality (e.g., two, three, more than three, etc.) oflight-emitting diodes. In the example of FIG. 7, each tile 38C includeslight-emitting diodes 38-1, 38-2, and 38-3. These light-emitting diodesmay have different properties to help tune the light emitted from thecell. In the example of FIG. 7, light-emitting diode 38-2 is larger thanlight-emitting diode 38-1, and light-emitting diode 38-1 is larger thanlight-emitting diode 38-3. Each light-emitting diode may have the sameor different sizes. The example of FIG. 7 in which each light-emittingdiode has a non-square rectangular shape is merely illustrative, andeach light-emitting diode may have any desired shape. In anotherpossible embodiment, the light-emitting diodes may have different peakwavelength emissions. Additionally, as shown in FIG. 2 eachlight-emitting diode may be covered by encapsulant 52. Eachlight-emitting diode in tile 38C may be covered by encapsulant ofdifferent shapes or encapsulant with different dopants.

In general, multiple light-emitting diodes in a given tile may havedifferent properties to tune the light that is emitted from that tile.These properties may include positioning, orientation, properties of thesolder attached to the light-emitting diodes, particle size ofencapsulant additives, and distribution of encapsulant additives. In oneillustrative example, light emitting diodes 38-1, 38-2, and 38-3 may bered, blue, and green light-emitting diodes respectively (and tile 38Cemits white light).

FIG. 2 shows light spreading layer 28 that is used to spread light fromlight-emitting diodes 38 in light-emitting diode array 36. FIG. 8 is across-sectional side view of an illustrative light-emitting diode arraywith a light redirecting layer on the printed circuit board adjacent thelight-emitting diodes. As shown in FIG. 8, light-emitting diodes 38 aremounted on an upper surface of printed circuit board 50. Lightredirecting layer 68 is also mounted on the upper surface of printedcircuit board 50. Light redirecting layer 68 may be a prismaticreflector that helps redirect light emitted from the edges oflight-emitting diodes 38. Layer 68 may be a separate film that islaminated to printed circuit board 50 or may be embossed on a UV curableor thermal curable coating on the printed circuit board.

Layer 68 may be designed to guide light in a particular direction toimprove optical uniformity and efficiency. The layer can includediffusive, refractive, and/or diffractive optical properties. Layer 68may include a micro structure, a parabolic reflector, an embossedstructure that diffracts a certain wavelength, or a prismatic structurethat emits light at a defined cone angle. Layer 68 may work incombination with light spreading layer 28 or may replace light spreadinglayer 28. In the example of FIG. 8, layer 68 includes two protrusionsbetween each adjacent light-emitting diode 38.

FIGS. 9 and 10 are cross-sectional side views showing how light-emittingdiodes 38 in light-emitting diode array 36 may or may not include areflector layer. As shown in FIG. 9, light-emitting diode may include areflector layer 70 (e.g., a distributed Bragg-reflector) to help directemitted light from light-emitting diode 38 sideways. This can reduce theon-axis intensity of light emitted by light-emitting diode 38. However,this increases the amount of light that hits the upper surface ofprinted circuit board 50, solder pads 54, and solder 56. Printed circuitboard 50, solder pads 54, and solder 56 may not have a high reflectance.Therefore, the presence of reflector layer 70 may result in high lightloss. To reduce light loss, the light-emitting diode may not includereflector layer 70 (as shown in FIG. 10).

FIG. 10 additionally shows various ways to reduce the solder thicknessand corresponding height of the light-emitting diode relative to theprinted circuit board and improve solder quality by reducing airenclosed under the light-emitting diode. As shown in FIG. 10, solderpads 54 may have recesses 53B (e.g., in the upper surface of the solderpads) that receive portions of solder 56. Recesses 53B allow solder 56to better attach to solder pad 54 due to capillarity and gravity.Similarly, the lower surface of light-emitting diode 38 may have one ormore recesses 53A that receive portions of solder 56. Again, recesses53A may allow solder 56 to better attach to light-emitting diode 38. Oneor more of the recesses may be coated with a conductive coating ifdesired.

Additionally, a coating 57 may be included on printed circuit board 50underneath light-emitting diode 38. Coating 57 may be a dielectric(e.g., non-conductive) coating. Coating 57 may be hydrophobic, allowingsolder 56 to flow across the coating. When solder 56 is deposited,excess solder may flow across coating 57 into recesses 55 in printedcircuit board 50. Recesses 55 may extend partially through printedcircuit board 50 or may pass entirely (e.g., from an upper surface to alower surface) through the circuit board. Vacuum suction may also beapplied (e.g., during solder deposition) to align the die and removeexcess air. Having excess solder flow into recesses 55 as described mayimprove solder quality and placement reliability, reduce die tilting,and reduce air under the die.

FIG. 11 shows ways to reduce light loss by increasing the reflectivityof the printed circuit board and other adjacent layers. As shown in FIG.11, solder mask 72 (sometimes referred to as a solder mask reflector ora reflective layer) may be included in light-emitting diode array 36.Solder mask 72 may be formed from a highly reflective material to reducelosses when light from light-emitting diode 38 hits solder mask 72. Thesolder mask may include titanium dioxide (TiO₂) (e.g., titanium dioxideparticles dispersed in a polymer) or other desired materials to increasereflectivity. The particle size of the titanium dioxide in the soldermask as well as the polymer refraction index may be tuned to optimizereflectivity. In one embodiment, the solder mask may be directlylaminated onto the upper surface of the printed circuit board. Inanother embodiment, the solder mask may be a thermally curable coatingor an ultraviolet light curable coating. The solder mask may have anydesired thickness (e.g., 50 microns, greater than 50 microns, less than100 microns, less than 50 microns, etc.).

Properties of printed circuit board 50 may also be optimized for highreflectivity. For example, glass fiber material or white polyimidepolymer may form the core of printed circuit board 50. The reflectivityof the solder mask and printed circuit board may be the same or may bedifferent (e.g., greater than 80%, greater than 50%, greater than 90%,greater than 92%, greater than 94%, between 85% and 95%, less than 90%,less than 99%, between 80% and 95%, etc.). Thermally conductive fillersmay also be added to printed circuit board 50 to tune the thermalconductivity of the printed circuit board.

There are numerous possible embodiments for the encapsulant thatencapsulates (e.g., conforms to) the light-emitting diodes on printedcircuit board 50. FIG. 12 is a cross-sectional side view of anillustrative light-emitting diode array with encapsulant 52 that has aplanar upper surface. As shown in FIG. 12, encapsulant 52 may have aplanar upper surface 74 that extends over the light-emitting diode. Theencapsulant 52 (sometimes referred to as a slab of encapsulant in thisembodiment) provides additional structural integrity for light-emittingdiode 38. Additionally, to reduce total internal reflection of lightemitted from light-emitting diode 38 off of the interface betweenencapsulant 52 and adjacent material 76 (e.g., air), encapsulant 52 mayhave an index of refraction that is between the index of refraction oflight-emitting diode 38 and material 76. For example, encapsulant 52 mayhave an index of refraction of between 1.2 and 1.5, between 1.3 and 1.4,less than 1.4, less than 1.5, greater than 1.1, greater than 1.2,greater than 1.3, about 1.35, etc. The slab of encapsulant 52 shown inFIG. 12 may cover all of the light-emitting diodes in light-emittingdiode array 36. Encapsulant 52 may have any desired thickness 78 (e.g.,between 0.1 and 0.4 millimeters, less than 0.5 millimeters, less than0.3 millimeters, between 0.2 and 0.4 millimeters, between 0.15 and 0.25millimeters, between 0.25 millimeters and 0.35 millimeters, about 0.2millimeters, about 0.3 millimeters, etc.).

FIG. 13 is a cross-sectional side view of an illustrative light-emittingdiode array with encapsulant 52 that has a curved upper surface. Asshown in FIG. 13, encapsulant 52 may have a curved upper surface 80(e.g., a convex upper surface). The encapsulant may form a dome shapeover the light-emitting diode. In this embodiment, the encapsulant maybe referred to as a droplet lens. The encapsulant 52 provides additionalstructural integrity for light-emitting diode 38. Additionally, toreduce total internal reflection of light emitted from light-emittingdiode 38 off of the interface between encapsulant 52 and adjacentmaterial 76 (e.g., air), encapsulant 52 may have an index of refractionthat is between the index of refraction of light-emitting diode 38 andmaterial 76. For example, encapsulant 52 may have an index of refractionof between 1.2 and 1.5, between 1.3 and 1.4, less than 1.4, less than1.5, greater than 1.1, greater than 1.2, greater than 1.3, about 1.35,etc. Each light-emitting diode may have a corresponding encapsulantportion (e.g., one dome of encapsulant for each light-emitting diode).Encapsulant 52 may have any desired thickness 82 (e.g., between 0.1 and0.4 millimeters, less than 0.5 millimeters, less than 0.3 millimeters,between 0.2 and 0.4 millimeters, between 0.15 and 0.25 millimeters,between 0.25 millimeters and 0.35 millimeters, about 0.2 millimeters,about 0.3 millimeters, etc.) and any desired width 84 (e.g., between 0.3and 2.5 millimeters, between 0.3 and 0.7 millimeters, between 0.8 and0.9 millimeters, greater than 0.5 millimeters, greater than 1.0millimeters, greater than 2.0 millimeters, less than 2.0 millimetersetc.). The ratio of width to thickness may be greater than 5-to-1,greater than 3-to-1, between 4-to-1 and 6-to-1, less than 10-to-1, etc.

FIG. 14 is a cross-sectional side view of an illustrative light-emittingdiode array with encapsulant 52 that has a curved upper surface and arecess. The encapsulant in FIG. 14 is similar to the encapsulant of FIG.13 (e.g., with a dome shape). However, in FIG. 14 encapsulant 52additional has a recess 86 formed over light-emitting diode 38. Recess86 may also be a referred to as a dip. Recess 86 may have any desiredshape (e.g., a pyramid shape). Recess 86 may further increase uniformitycompared to the embodiment of FIG. 13 (without the recess) by reducingthe intensity of on-axis (e.g., zero order) light.

FIGS. 15-17 show examples of a dopant being included in encapsulant 52.FIG. 15 is a cross-sectional side view showing a light-emitting diode 38covered by encapsulant 52 that includes dopant 88. Dopant 88 (sometimesreferred to as additive 88) may be a scattering dopant. The presence ofdopant 88 may increase the light-emitting area of light-emitting diode38. Without the scattering dopant, light emitted from light-emittingdiode 38 will be concentrated directly over light-emitting diode 38. Thescattering dopant, however, may cause light from light-emitting diode 38to be spread throughout encapsulant 52. Therefore, the entire area ofencapsulant 52 emits light from light-emitting diode 38. This spreadingof light may work in combination with or replace light spreading layer28 in FIG. 2, for example.

Any desired dopants may be used for dopant 88. For example, dopant 88may include polymer nanoparticles, inorganic nanoparticles, voids (e.g.,air bubbles), or any other desired dopant. Distribution of the dopantsin encapsulation 52 can be controlled via buoyancy of the dopant in theencapsulation material (e.g., encapsulation resin). For example, FIG. 15shows a neutral buoyancy example. Dopants 88 in FIG. 15 are thereforedistributed evenly throughout encapsulation 52 (e.g., dopants may besuspended anywhere in the encapsulation material). FIG. 16 shows anexample where dopants 88 have high buoyancy and therefore tend to bedistributed along the upper surface of the encapsulant (e.g., becausethe dopants float to the top of the encapsulant). In contrast, FIG. 17shows an example where dopants 88 have a low buoyancy and therefore tendto be distributed along the lower surface of the encapsulant (e.g.,because the dopants sink to the bottom of the encapsulant).

The location of dopants 88 may be controlled if desired. For example,dopants may be placed directly on top of light-emitting diode 38.Dopants 88 may also be designed such that the orientation of dopants 88in encapsulant 52 can be controlled. In one example, shown in FIG. 18,dopant 88 (e.g., a particular particle) may have two portions with twodifferent densities. As shown in FIG. 18, dopant 88 may include portion88-1 and portion 88-2. Portion 88-2 may have a different density (e.g.,higher) than portion 88-1. In another example, shown in FIG. 19, theshape of dopant 88 may be designed to control the orientation of dopant88. The shape shown in FIG. 19 (sometimes referred to as a mushroomshape) may allow control of the orientation of dopant 88 in encapsulant52.

The aforementioned examples showing dopants 88 in a dome-shapedencapsulant region are merely illustrative. In general, any desiredencapsulant (e.g., encapsulant with a planar upper surface as in FIG. 12or a recessed portion as in FIG. 14) may include the dopants.

FIG. 20 is a cross-sectional side view of an illustrative backlight unitshowing how light-emitting diode array 36 may include a patterned layer90 on encapsulation 52. Patterned layer 90 may help spread light fromlight-emitting diode 38 (e.g., working in combination with or replacinglight spreading layer 28 in FIG. 2). Patterned layer 90 may also helpmaintain gap 92 (e.g., an air gap) between optical films 26 andencapsulant 52. Patterned layer 90 may be a separate film that isattached to an upper surface of encapsulant 52. Alternatively, patternedlayer 90 may be formed from a patterned portion of the encapsulantmaterial (e.g., patterned layer 90 may be part of encapsulant 52).

FIG. 21 is a cross-sectional side view of an illustrative backlight unitshowing how light-emitting diode array 36 may include encapsulation 52having a textured surface. As shown in FIG. 21, encapsulation 52 has atextured upper surface 94. Textured upper surface 94 may be generallyplanar (e.g., the encapsulation is formed as a slab). However, the uppersurface may be rough (instead of smooth). The textured upper surface mayprevent total internal reflection of light emitted from light-emittingdiode 38, increasing efficiency. The texture of the upper surface may berandom.

To increase the reflectivity of the printed circuit board in a backlightunit, a reflective layer may be used. For example, a reflective materialmay be laminated on the printed circuit board. However, care must betaken to ensure that the reflective material is not damaged duringsolder reflow. The reflective material may have a melting point lessthan or equal to that of the solder. Reflowing the solder may thereforemelt (and damage) the reflective material.

To prevent damage of the reflective layer, the reflective layer may belaminated to the circuit board after solder reflow. Alternatively, thereflective layer may be attached to the printed circuit board beforesolder reflow. The solder mask may then be used to protect thereflective layer from damage during solder reflow. An arrangement ofthis type is shown in FIG. 22.

As shown in FIG. 22, reflective layer 96 may be formed on printedcircuit board 50 (e.g., on an upper surface of printed circuit board50). The reflective layer may be formed from any desired material. Thereflective layer may be formed from one or more layers. The reflectivelayer may have a high reflectance of light at the wavelength emitted bythe light-emitting diodes. For example, if the light-emitting diodesemit blue light, reflective layer 96 may have a high reflectance (e.g.,greater than 80%, greater than 90%, greater than 92%, greater than 94%,greater than 96%, less than 99%, etc.) of the blue light. Solder 56 mayalso be formed on printed circuit board 50 (e.g., on conductive pads onan upper surface of the printed circuit board that may not be covered bythe reflective layer). Heat source 100 may be used to heat solder 56 forreflow. Heat source 100 may heat solder 56 by emitting energy (e.g.,infrared light) through solder mask 98. Solder mask 98 may have openingsthat overlap solder 56. Portions of printed circuit board 50 that arenot covered by solder 56 may be covered by solder mask 98. In this way,solder mask 98 may prevent reflective layer 96 from being damaged duringheating of solder 56. Solder mask 98 may be formed from any materialthat is opaque or resistant to the energy (102) emitted by heat source100. Heat source 100 may, for example, be a laser that emits infraredlight.

In certain embodiments, backlight unit 42 may include a polarizationgrating that helps increase the point spread function (PSF) width of thelight emitted by light-emitting diodes 38. Increasing the PSF width mayreduce reliance on the reflectance of the printed circuit board forrecycling. FIGS. 23-25 are cross-sectional side views of illustrativebacklight units with a polarization grating layer (sometimes referred toas a diffractive layer, light spreading layer, polarization phasegrating layer, polarization grating, or polarization phase grating). Asshown in FIG. 23, backlight unit 42 includes light-emitting diodes 38 onprinted circuit board 50 that are covered by encapsulation 52. Thebacklight unit may also have optical films 26 that include lightspreading layer 28. However, the backlight unit may include additionallight spreading layer 104. Polarization grating layer 104 (sometimesreferred to as a diffractive layer, light spreading layer, polarizationphase grating layer, polarization grating, or polarization phasegrating) may split light received from light-emitting diodes 38 into twodifferent polarizations. Light spreading layer 104 may reduce theon-axis intensity peak associated with the light-emitting diodes.

As shown in FIG. 23, light spreading layer 104 may be coupled toencapsulation 52 that covers the light-emitting diodes (e.g., lightspreading layer 104 may directly contact encapsulation 52). This exampleis merely illustrative. In another example shown in FIG. 24, lightspreading layer 104 is formed on a lower surface of light spreadinglayer 28. Light spreading layer 28 and light spreading layer 104 may beattached together in any desired manner. Light spreading layer 28 mayhave evenly patterned protrusions on an upper surface of the lightspreading layer. In yet another embodiment, shown in FIG. 25, a waveguide layer 106 (sometimes referred to as a light guiding layer) may beinterposed between light spreading layer 28 and light spreading layer104. Light spreading layer 28 may have protrusions on an upper surfaceof the light spreading layer that are unevenly spaced (e.g.,statistically spaced to control wave guide spreading) in thisembodiment.

Other structures may be used to increase the point spread function (PSF)width of the light emitted by light-emitting diodes 38. FIGS. 26 and 27are cross-sectional side views of backlight units that includestructures to increase the point spread function (PSF) width of thelight emitted by light-emitting diodes 38 in combination with a highlyreflective printed circuit board. As shown in FIG. 26, light-emittingdiodes 38 may be positioned on printed circuit board 50 and may becovered by encapsulant 52. Each light-emitting diode 38 may be coveredby a respective portion of encapsulant that has a respective parabolicupper surface 108. The parabolic upper surfaces of adjacent encapsulantportions may meet at vertex 110. This portion may be referred to as adimpled or recessed portion of encapsulant 52.

Light 112 emitted from light-emitting diodes 38 may be reflected towardsvertex 110 by total internal reflection (TIR). When light 112 reachesvertex 110 (sometimes referred to as light leakage promotion structure110), the light may be more likely to exit encapsulant 52 in thedirection of the viewer. In this way, light from the light-emittingdiodes may be directed to and emitted from the area between adjacentlight-emitting diodes (thereby increasing the PSF width). This type ofarrangement relies on the light reflecting off of printed circuit board50 (e.g., light is emitted from the light-emitting diode, reflects offof the parabolic upper surface of the encapsulant due to total internalreflection, and then reflects off of the upper surface of the printedcircuit board). Therefore, if the reflectance of the printed circuitboard is low, the efficiency of a backlight unit with the arrangement ofFIG. 26 may be low. The arrangement of FIG. 26 may therefore be suitedto embodiments where the printed circuit board has a high reflectance(e.g., greater than 90%, greater than 94%, greater than 96%, etc.).

In another embodiment, shown in FIG. 27, light-emitting diodes 38 areagain positioned on printed circuit board 50 and may be covered byencapsulant 52. In this embodiment, the light-emitting diodes arecovered by encapsulant with a planar upper surface (e.g., a slab ofencapsulant). A light leakage promotion structure 114 may be interposedbetween each light-emitting diode. Light leakage promotion structure 114in FIG. 27 may be formed from an opaque white structure. In anotherembodiment, light leakage promotion structure 114 may be an air gap(sometimes referred to as a void) in encapsulant 52.

Light emitted from light-emitting diodes 38 may be reflected towardslight leakage promotion structure 114 by total internal reflection(TIR). When light reaches light leakage promotion structure 114, thelight may be more likely to exit encapsulant 52 in the direction of theviewer. In this way, light from the light-emitting diodes may bedirected to and emitted from the area between adjacent light-emittingdiodes (thereby increasing the PSF width). This type of arrangementrelies on the light reflecting off of printed circuit board 50 (e.g.,light is emitted from the light-emitting diode, reflects off of theupper surface of the encapsulant due to total internal reflection, andthen reflects off of the upper surface of the printed circuit board).The arrangement of FIG. 27 may therefore be suited to embodiments wherethe printed circuit board has a high reflectance (e.g., greater than90%, greater than 94%, greater than 96%, etc.).

There are many possible ways for light spreading layer 28 (shown in FIG.2, for example) to spread light. FIG. 28 is a cross-sectional side viewof an illustrative display with a light spreading layer that hasmicrolens structures on both the upper surface and lower surface. Asshown in FIG. 28, light spreading layer 28 has an upper surface 116 anda lower surface 118. Upper surface 116 has a plurality of microlenses120 that are formed from recesses in the upper surface of the lightspreading layer. Microlenses 120 may be formed from spherically shapedrecesses or recesses of any other desired shape in upper surface 116.Lower surface 118 also has a plurality of microlenses 122 that areformed from recesses in the lower surface of the light spreading layer.Microlenses 122 may be formed from spherically shaped recesses orrecesses of any other desired shape in lower surface 118. Microlenses122 may be larger than microlenses 120 (as shown in FIG. 28). However,this example is merely illustrative and microlenses 122 may also be thesame size or smaller than microlenses 120. Additionally, each microlensin microlens array 120 may have the same size or a different size, andeach microlens in microlens array 122 may have the same size or adifferent size.

Another arrangement for light spreading layer 28 is shown in FIG. 29. Asshown in FIG. 29, the light spreading layer may include two films(layers). In the example of FIG. 29, light spreading layer 28-1 ispositioned above light spreading layer 28-2. Light spreading layer 28-1and light spreading layer 28-2 may be attached using adhesive 128 (e.g.,pressure sensitive adhesive), for example. Adhesive 128 may havediffusive properties if desired. Alternatively, light spreading layers28-1 and 28-2 may be laminated together to form an integral film.

As shown in FIG. 29, light spreading layer 28-2 has an upper surfacewith microlenses 132. Microlenses 132 may be formed from a plurality ofrecesses in the upper surface of light spreading layer 28-2. Lightspreading layer 28-2 may also include a plurality of protrusions 130.Protrusions 130 may protrude towards the light-emitting diodes.Protrusions 130 may be elongated protrusions (sometimes referred to asridges) that extend along a longitudinal axis across the layer (e.g.,parallel to the Y-axis in FIG. 29). Light spreading layer 28-1 may bethe same as light spreading layer 28-2, except for being rotated 90°relative to light spreading layer 28-2. For example, light spreadinglayer 28-1 also has a plurality of protrusions that extend along alongitudinal axis across the layer. However, the protrusions of lightspreading layer 28-1 may extend parallel to the X-axis (perpendicular tothe protrusions of light spreading layer 28-2). Light spreading layer28-1 also includes microlenses 126. Microlenses 126 may be formed from aplurality of recesses in the upper surface 124 of light spreading layer28-1.

Light emitted from light-emitting diodes 38 will be spread relative to afirst axis (e.g., from a point source to two points) upon reachinglight-spreading layer 28-2. Microlenses 132 may reduce total internalreflection to promote light passing to light spreading layer 28-1. Lightspreading layer 28-1 will spread the incoming light relative to a secondaxis (e.g., from the two points to four points) that is perpendicular tothe first axis (because the protrusions of layer 28-1 are perpendicularto the protrusions of layer 28-2). Microlenses 126 may reduce totalinternal reflection to promote leakage of light out of layer 28-1towards layer 30. Layers 28-1 and 28-2 may each have any desiredthickness (e.g., between 20 and 30 microns, between 20 and 25 microns,less than 50 microns, less than 25 microns, about 22 microns, greaterthan 15 microns, less than 100 microns, etc.). The thickness of layers28-1 and 28-2 may be the same.

The example of light spreading layer 28 having elongated protrusionsthat extend along a longitudinal axis (and that split an incoming pointlight source into two points) is merely illustrative. If desired, lightspreading layer 28 may instead include an array of protrusions orrecesses. Each protrusion may split an incoming point light source intothree or more points. FIG. 30 is a cross-sectional side view of a lightspreading layer 28 that includes protrusions 134. Protrusions 134 mayhave any desired shape. For example, the protrusions may have apyramidal shape (e.g., with a square base and four triangular faces thatmeet at vertex 136) or a triangular pyramidal shape (e.g., with atriangular base and three triangular faces that meet at vertex 136). Thepyramidal protrusions may split a point light source into four points,whereas the triangular pyramidal protrusions may split a point lightsource into three points. In another embodiment, shown in FIG. 31, lightspreading layer 28 may include a plurality of recesses 138. Recesses 138may have any desired shape. For example, the recesses may have apyramidal shape (e.g., with a square base and four triangular faces thatmeet at vertex 140) or a triangular pyramidal shape (e.g., with atriangular base and three triangular faces that meet at vertex 140). Thepyramidal recesses may split a point light source into four points,whereas the triangular pyramidal recesses may split a point light sourceinto three points. FIG. 32 is a top view of light spreading layer 28showing how protrusions 134 (or recesses 138) may be arranged in anarray. The protrusions and/or recesses may have any desired shapes andmay be arranged in any desired type of array.

FIG. 33 is a top view of a light spreading layer with protrusions 134formed from partial-cube structures (sometimes referred to ascorner-cubes). As shown, each protrusion (or recess) may be formed bythree square faces (e.g., a corner cube) that meet at a vertex 136. InFIG. 34, each protrusion 134 is formed from two partial-cube structuresthat are off-set from each other. Each protrusion 134 in FIG. 34therefore has six faces (and splits an incoming point light source intosix points).

In yet another embodiment shown in FIG. 35, protrusion 134 may be formedfrom a tapered pyramid structure. As shown in FIG. 35, the protrusionmay have four faces 142 that meet at vertex 148. Each face may have alower portion 144 and an upper portion 146 that are at an angle 150relative to each other. Angle 150 may be selected to optimize the lightspreading functionality of the light spreading layer. Two films with anyof the light spreading features shown in FIGS. 30-35 may be laminatedtogether if desired (e.g., a first film with protrusions on an uppersurface may be laminated to a second film with protrusions on a lowersurface).

Additional embodiments for spreading light from light-emitting diodes 38are shown in FIG. 36-39. FIG. 36 is a cross-sectional side view of anillustrative backlight unit with a microlens array formed on a lowersurface of partially reflective layer 30. Microlenses 152 may protrudefrom the lower surface of partially reflective layer 30. The microlensesmay help spread light received from light-emitting diodes 38.Microlenses 152 may have an associated focal length (1). The thickness154 of encapsulant 52 may be a function of the focal length (e.g., t=n×fwhere t is thickness 154 and n is the refractive index of theencapsulant material).

FIG. 37 shows an arrangement similar to that of FIG. 36. However, inFIG. 36 encapsulant 52 is formed across all of the light-emitting diodesin the array (e.g., encapsulant slab). In FIG. 37, each light-emittingdiode has a respective portion of encapsulant 52. In FIG. 37, thethickness 154 of each encapsulant portion may still be selected based onthe focal length of microlenses 152 and the refractive index of theencapsulant material.

FIG. 38 shows yet another embodiment with microlenses 152 on the lowersurface of partially reflective layer 30. In FIG. 38, eachlight-emitting diode may be covered by an encapsulant portion 52 havinga curved upper surface (instead of a planar upper surface as in FIG.37). In FIG. 38, the thickness 154 of each encapsulant portion may beselected based on the focal length of microlenses 152 (e.g., t=f where tis thickness 154 and f is the focal length of microlenses 152).

The example in FIGS. 36-38 of microlenses 152 being formed on a lowersurface of partially reflective layer 30 is merely illustrative.Microlenses 152 may be formed in other locations in the display ifdesired. FIG. 39 shows an embodiment where microlenses 152 are formed onsubstrate 156 (separate from partially reflective layer 30). There maybe an air gap 158 between light-emitting diodes 38 and the substrate 156for microlenses 152.

In the embodiments shown in FIGS. 36-39, light spreading layer 28 isomitted (because microlenses 152 spread the light from thelight-emitting diodes). However, this is merely illustrative. Ifdesired, light spreading layer 28 may be included in any of theembodiments of FIGS. 36-39 (and spread light in combination withmicrolenses 152).

FIG. 2 showed an example where backlight unit 42 includes a brightnessenhancement film 46 and a collimating layer 44. Examples of these layersare again shown in FIG. 40. As shown in FIG. 40, brightness enhancementfilm 46 may include protrusions 160 on an upper surface of the film.Collimating layer 44 may include an array of microlenses 162 on a lowersurface of the film. To reduce the number of layers in backlight unit42, brightness enhancement film 46 and collimating layer 44 may becombined into a single film. FIG. 41 is a cross-sectional side view ofan illustrative optical layer 164 having protrusions 160 on an uppersurface (serving the function of the brightness enhancement film) andmicrolenses 162 on a lower surface (serving the function of thecollimating layer).

Light leaking from the edge of optical films 26 may cause the edge ofthe display to have an undesirable blue tint (due to the pixel array 24receiving an excess of blue light from light-emitting diodes 38). Tomitigate this problem, one or more coatings may be included in the edgeof the display. As shown in FIG. 42, display 14 may include a coating168 on a lower layer of pixel array 24. The display may include achassis 166 (e.g., a plastic chassis sometimes referred to as ap-chassis) that supports one or more other layers in display 14. One ormore surfaces of the chassis (e.g., an edge surface 176) is coated bycoating 174. An upper surface of the optical layers is coated withcoating 170. An edge surface of the optical layers is coated withcoating 172. Coatings 168, 170, 172, and 174 (which are all in the edgeof the display) may be formed from a black material, a gray material, ayellow material, or a phosphor material. The black material, graymaterial, and yellow material may adjust or eliminate reflectance ofblue light (thereby reducing the excess blue light in the edge of thedisplay). The phosphor material may convert blue light to white light(thereby reducing the excess blue light in the edge of the display).

In some displays (e.g., as shown in FIG. 3), the light-emitting diodesfor the display may be arranged in a grid. The light-emitting diodes maybe arranged in evenly spaced rows and columns that extend along theentire display. This type of arrangement may be referred to as a squaregrid or a rectangular grid. However, having the light-emitting diodesarranged in a rectangular grid may result in visible artifacts(sometimes referred to as grid mura) when operating the display. Tomitigate visible artifacts associated with the positioning of thelight-emitting diodes in a grid, the positions of the light-emittingdiodes may be dithered. In other words, the actual position of eachlight-emitting diode may be adjusted relative to the rectangular-gridposition for that light-emitting diode.

FIG. 43 is a diagram showing how the positions of light-emitting diodesmay be dithered to improve performance of the display. FIG. 43 showsrectangular grid positions 202-1, 202-2, 202-3, and 202-4. Therectangular grid positions may be positions associated with a regularlyspaced grid that extends across the entire display (e.g., eachrectangular grid position is part of a row of rectangular grid positionsthat extends across the entire display and each rectangular gridposition is part of a column of rectangular grid positions that extendsacross the entire display). Light-emitting diodes may optionally bemounted at each rectangular grid position (similar to as in FIG. 3).Alternatively, to mitigate visible artifacts caused by this arrangementthe light-emitting diodes may be offset from the rectangular gridpositions as shown in FIG. 43.

In FIG. 43, each light-emitting diode may be positioned at a positionthat is offset relative to its corresponding rectangular grid position.For example, a light-emitting diode may be located at position 206-1that is moved by offset distance 204-1 away from rectangular gridposition 202-1. Offset distance 204-1 is at an angle 208-1 relative tothe X-axis. Similarly, a light-emitting diode may be located at position206-2 that is moved by offset distance 204-2 away from rectangular gridposition 202-2. Offset distance 204-2 is at an angle 208-2 relative tothe X-axis. A light-emitting diode may be located at position 206-3 thatis moved by offset distance 204-3 away from rectangular grid position202-3. Offset distance 204-3 is at an angle 208-3 relative to theX-axis. A light-emitting diode may be located at position 206-4 that ismoved by offset distance 204-4 away from rectangular grid position202-4. Offset distance 204-4 is at an angle 208-4 relative to theX-axis.

Each offset distance may have any desired magnitude. In one example, theoffset distance may be greater than 0, may be shorter than a distancebetween adjacent rectangular grid positions in the same row (e.g.,distance 210), and may be shorter than a distance between adjacentrectangular grid positions in the same column (e.g., distance 212). Eachoffset distance may be the same (e.g., offset distances 204-1, 204-2,204-3, and 204-4 may all have the same magnitude) or one or more of theoffset distances may be different (e.g., offset distances 204-1, 204-2,204-3, and 204-4 may all have different magnitudes). In some cases, theoffset distances may be random or pseudo-random.

Similarly, each offset distance may be moved away from its correspondingrectangular grid position by any desired angle. In one illustrativeexample, each angle is 90 degrees offset from the angle associated withthe light-emitting diode in an adjacent row or column. As shown in FIG.43, angle 204-4 may be a given angle (e.g., θ) between 0 degrees and 90degrees, angle 204-2 may be offset by 90 degrees relative to the givenangle (e.g., angle 204-2=θ+90 degrees), angle 204-1 may be offset by 180degrees relative to the given angle (e.g., angle 204-1=θ+180 degrees),and angle 204-3 may be offset by 270 degrees relative to the given angle(e.g., angle 204-3=θ+270 degrees). Effectively, the 2×2 group oflight-emitting diodes is rotated relative to the rectangular gridpositions. Each 2×2 group of light-emitting diodes within the displaymay be independently rotated. The example of using a 2×2 group is merelyillustrative. Light-emitting diode groups of any desired size may berotated across the display.

Each light-emitting diode position 206 may be offset from itsrectangular grid position 202 by an angle that is 180 degrees differentthan the light-emitting diode position of a diagonally adjacentlight-emitting diode. Each light-emitting diode position 206 may beoffset from its rectangular grid position 202 by an angle that is 90degrees different than the light-emitting diode position of ahorizontally adjacent light-emitting diode (e.g., a light-emitting diodein the same row). Each light-emitting diode position 206 may be offsetfrom its rectangular grid position 202 by an angle that is 90 degreesdifferent than the light-emitting diode position of a verticallyadjacent light-emitting diode (e.g., a light-emitting diode in the samecolumn).

This example is merely illustrative. In general, the location of eachlight-emitting diode and the rectangular grid position associated withthat light-emitting diode may be offset by any desired angle. In somecases, the angles may be random or pseudo-random.

FIG. 44 is a top view of an illustrative display with illustrativelight-emitting diodes that are dithered. As shown, each light-emittingdiode 38 is dithered with respect to a rectangular grid position (asshown in FIG. 43). In particular, each 2×2 group of light-emittingdiodes are rotated relative to their rectangular grid positions.Therefore, the light-emitting diodes are not arranged in uniform rowsand columns that extend across the entire display. Dithering thelocation of the light-emitting diodes in this way mitigates grid muraassociated with the light-emitting diodes in a uniform rectangular grid.

Each light-emitting diode is covered by a respective encapsulant 52(e.g., a slab or dome of encapsulant). In FIG. 44, each light-emittingdiode is centered underneath its respective encapsulant layer 52.However, this example is merely illustrative. If desired, eachlight-emitting diode may be offset relative to the center of theoverlying encapsulant layer.

FIG. 45 is a top view of an illustrative display with light-emittingdiodes that are offset relative to the center of the overlyingencapsulant layer. Similar to how the positions of the light-emittingdiodes are shifted relative to the rectangular grid positions in FIG.43, the positions of the encapsulant are shifted relative to thepositions of the light-emitting diodes in FIG. 45. As shown, eachencapsulant layer 52 has a center that is shifted relative to thelight-emitting diode 38. The shifting of the center of the encapsulantlayer in FIG. 45 may follow the same offset scheme as shown in FIGS. 43and 44 in connection with the position of the light-emitting diodes.

When the encapsulant layers are shifted relative to the light-emittingdiodes, the light-emitting diodes may be arranged in a rectangular grid(as shown in FIG. 45). Alternatively, the positions of thelight-emitting diodes may be dithered (as in FIG. 44) and theencapsulant layers may be offset relative to the light-emitting diodepositions.

In another embodiment, shown in FIG. 46, light-emitting diodes 38 arepositioned on printed circuit board 50 and may be covered by encapsulant52. In this embodiment, the light-emitting diodes include a reflectorlayer 70 (e.g., a distributed Bragg-reflector) to help direct emittedlight from light-emitting diode 38 sideways. This can reduce the on-axisintensity of light emitted by light-emitting diode 38. As shown in FIG.46, due to the presence of reflector layer 70, the cone of light emittedby each light-emitting diode 38 may have a width 222. The width of thecone of high intensity light may be relatively narrow (e.g., width 224),which helps promote uniform light distribution across the display.

FIG. 46 also shows how each light-emitting diode is covered byencapsulant with a planar upper surface (e.g., a slab of encapsulant).In one arrangement, the thickness 226 of the encapsulant may be selectedbased on the thickness 228 of the light-emitting diode 38. In otherwords, a ratio between the thickness of the encapsulant and thethickness of the light-emitting diode may be selected to optimizedisplay performance. The ratio between the thickness of the encapsulantand the thickness of the light-emitting diode may be used to help spreadlight evenly across the display. For example, if thickness 226 ofencapsulant 52 is larger, the emission area associated withlight-emitting diode 38 will tend be larger. If thickness 226 ofencapsulant 52 is smaller, the emission area associated withlight-emitting diode 38 will tend to be smaller. The thickness maytherefore be selected such that the emission areas of adjacentlight-emitting diodes meet at an interface without overlapping andwithout having an emission-free gap between them. The ratio of thickness226 to thickness 228 may be between 3:1 and 10:1, between 2:1 and 20:1,between 5:1 and 15:1, less than 10:1, more than 10:1, less than 5:1,more than 5:1, more than 20:1, less than 8:1, etc.

In cases where encapsulant is formed as a slab over the light-emittingdiodes (as in FIG. 46), the slab may be susceptible to warping. Thelight-emitting diodes generate heat that may heat the encapsulant slab.The encapsulant slab may expand and contract with temperature variationsassociated with operation of the light-emitting diodes due to thermalexpansion. To maintain the structural integrity of the encapsulant slab,a support structure may be incorporated into the display.

FIG. 47 is a cross-sectional side view of an illustrative display thatincludes a support structure 232 to maintain the structural integrity ofencapsulant slab 52. Support structure 232 may have a low coefficient ofthermal expansion (e.g., lower than the encapsulant) to ensure it is notadversely affected by temperature changes associated with operation ofthe light-emitting diodes. Support structure 232 may not affect thelight emitted from the light-emitting diodes. In other words, supportstructure 232 may be formed from a transparent material that has thesame index-of-refraction as the surrounding encapsulant slab 52 (e.g.,support structure 232 is index-matched to slab 52). Support structure232 will therefore not influence the path or intensity of the lightemitted from light-emitting diodes 38. Support structure 232 maytherefore have any desired shape (since the shape will not affect theoptical performance of the display).

FIG. 48 is a cross-sectional side view of an illustrative display with asupport structure that also serves as a light-leakage promotionstructure. Similar to as in FIG. 47, support structure 234 in FIG. 48may be used to maintain the structural integrity of encapsulant slab 52.Support structure 234 may have a low coefficient of thermal expansion(e.g., lower than the encapsulant) to ensure it is not adverselyaffected by temperature changes associated with operation of thelight-emitting diodes. However, unlike in FIG. 47, the support structure234 in FIG. 48 may reflect light from light-emitting diodes 38. Supportstructure 234 may be formed from a reflective material (e.g., a whitestructure, metal structure, etc.). Alternatively, support structure 234may be formed from a transparent material that has anindex-of-refraction that is different than the index-of-refraction ofencapsulant layer 52. The index-of-refraction difference may besufficient for total internal reflection (TIR) to occur when light fromlight-emitting diodes 238 reaches support structure 234. Supportstructure 234 may have a shape that is selected to redirect lightupwards towards the viewer. The example of FIG. 48 of support structure234 having a triangular cross-sectional shape is merely illustrative.Support structure 234 may have a pyramidal shape, cone shape, domeshape, etc.

As previously mentioned, display 14 may include a color conversion layer34. Color conversion layer 34 may convert the light from LEDs 38 from afirst color to a different color. For example, when the LEDs emit bluelight, color conversion layer 34 may include a phosphor layer 40 (e.g.,a layer of white phosphor material or other photoluminescent material)that converts blue light into white light. If desired, otherphotoluminescent materials may be used to convert blue light to light ofdifferent colors (e.g., red light, green light, white light, etc.). Forexample, one layer 34 may have a phosphor layer 40 that includes quantumdots that convert blue light into red and green light (e.g., to producewhite backlight illumination that includes, red, green, and bluecomponents). FIG. 49 is a cross-sectional side view of an illustrativecolor conversion layer.

As shown in FIG. 49, color conversion layer 34 includes phosphor layer40 with red quantum dots 242 and green quantum dots 244. The red quantumdots may convert blue light from the light-emitting diodes of thedisplay into red light whereas the green quantum dots may convert bluelight from the light-emitting diodes of the display into green light. Apartially reflective layer 41 (sometimes referred to as a dichroic layeror dichroic filter layer) may optionally be included in the colorconversion layer. The dichroic filter 41 may reflect all red and greenlight and partially reflect blue light, for example. An additional filmsuch as film 246 may also be included in the color conversion layer. Theadditional film 246 (sometimes referred to as an optical film) may beformed from a polymer material (e.g., polyethylene terephthalate).

As blue light from the light-emitting diodes passes through phosphorlayer, the blue light is converted to red and green light by quantumdots 242 and 244. The longer the distance the blue light travels throughphosphor layer 40, the more likely it is that the blue light will beconverted to red and green light. This may result in undesirable displayperformance. As shown in FIG. 49, on-axis light 248 (e.g., light that isparallel or close to parallel to the Z-axis) has an optical path with alength 252 in phosphor layer 40. In contrast, off-axis light 250 (e.g.,light at a relatively large angle relative to the Z-axis) has an opticalpath with a length 254 in phosphor layer 40. As shown, the optical path254 is longer than optical path 252. Accordingly, the off-axis lightwill include more red and green light and less blue light (because moreblue light is converted while in the phosphor layer for a longer time)than the on-axis light. This may lead to non-uniformity in the display.

There are numerous ways to prevent non-uniformity in the display due toa narrower angular profile of blue light than red and green light. Inone illustrative embodiment, shown in FIG. 50A, reflective structuresmay be introduced in the phosphor layer to reduce the length of theoptical path of off-axis light. Structures 256 (sometimes referred to asmicrostructures or path-length-reducing structures) may not include anyquantum dots 242 or 244. Structures 256 may be formed from any desiredreflective materials. Structures 256 may have a reflectance that isgreater than 40%, may have a reflectance that is greater than 30%, mayhave a reflectance that is greater than 50%, or may have any otherdesired reflectance. Structures 256 reduce the optical pathlength ofoff-axis light through phosphor layer 40, making the on-axis andoff-axis light more uniform. The smallest dimension of each structure256 may be less than 100 microns, less than 10 microns, less than 5microns, less than 2 microns, less than 1 micron, less than 0.1 micron,greater than 0.1 micron, greater than 1 micron, etc. The thickness 249of structures 256 may be less than the thickness of phosphor layer 40(as in FIG. 50A) or may be equal to the thickness of phosphor layer 40.A distance 247 may separate structures 256. Distance 247 may be greaterthan 15 microns, greater than 10 microns, greater than 5 microns,greater than 25 microns, less than 50 microns, or any other desireddistance.

In FIG. 50A, reflective structures 256 are formed in phosphor layer 40.In other words, the reflective structures 256 are embedded in and indirect contact with phosphor layer 40. However, this example is merelyillustrative. Reflective structures 256 may instead be immersed in atransparent coating layer and then laminated to the phosphor layer, asshown in FIG. 50B.

FIG. 50B shows a cross-sectional side view of color conversion layer 34in which reflective structures 256 are formed in an additional layer245. Additional layer 245 is interposed between phosphor layer and film246. Additional layer 245 may include reflective structures 256 embeddedin a transparent coating 251 (sometimes referred to as filler material).Alternatively, filler material 251 may be air. In embodiments wherefiller material 251 is air, an additional transparent coating 253 may beinterposed between the reflective structures and the phosphor layer, asshown in FIG. 50C.

FIG. 51A is a top view of a phosphor layer with reflective structuressuch as the phosphor layer of FIG. 50A. As shown in FIG. 51A, structures256 may have a hexagonal cross-sectional shape when viewed from above.This example is merely illustrative. Structures 256 may have any desiredcross-sectional shape when viewed from above (e.g., circular, square,non-square rectangular, octagonal, etc.). Structures 256 may alsooptionally be cones, pyramids, or other shapes having a varyingcross-section along the Z-axis.

In another illustrative embodiment, shown in FIG. 51B, the reflectivestructures may be arranged in a honeycomb pattern. In other words, thereflective structures may be arranged in an interconnected web thatdefines hexagonal openings (filled by phosphor layer 40, for example).This example is merely illustrative. The reflective structures may bearranged in an interconnected web that defines a plurality of triangularopenings, a plurality of circular openings, a plurality of squareopenings, a plurality of octagonal openings, a plurality of non-squarerectangular openings, etc. In one embodiment, the reflective structures(sometimes referred to as reflective walls) may have openings to allowthe phosphor layer to flow between cavities. Optional openings 265 inthe reflective walls are shown in FIG. 51B. Each reflective wall portionmay form one of six sides of the hexagonal shape around a respectivecell of phosphor material. Each reflective wall portion may have onerespective opening, each reflective wall portion may have two or morerespective openings, or only some of the reflective wall portions mayhave openings.

FIG. 52 is a cross-sectional side view of an illustrative colorconversion layer with Rayleigh scattering dopants for increasing theamount of off-axis blue light. As shown in the inset portion of FIG. 52,red quantum dots 242 output light in a random direction (e.g., thedirection that red light is output is not correlated to the directionthat blue light is received). Similarly, green quantum dots 244 outputlight in a random direction (e.g., the direction that green light isoutput is not correlated to the direction that blue light is received).To make the emission direction of blue light more random (and thereforeequalize the off-axis emission of blue light to the off-axis emission ofred and green light), Rayleigh scattering dopants 262 may be included inthe phosphor layer. Rayleigh scattering dopants 262 may elasticallyscatter blue light. This means that no energy is lost when the Rayleighscattering dopants 262 receive blue light and that the wavelength of thelight is not changed by the Rayleigh scattering dopants. However, theRayleigh scattering dopants randomize the direction of the blue light.The blue light will be scattered by the Rayleigh scattering dopantswhile the red and green light will tend not to be scattered by theRayleigh scattering dopants. Consequently, the distribution of red,blue, and green light may be equalized both on-axis and off-axis.

The average diameter of the Rayleigh scattering dopants may be between 5and 20 nanometers, less than 100 nanometers, less than 50 nanometers,less than 20 nanometers, more than 5 nanometers, more than 1 nanometer,or any other desired diameter. The average diameter of quantum dots 242and 244 may be more than 1 micron, more than 2 microns, between 1 and 3microns, less than 5 microns, or any other desired diameter.

Uniformity of the display may also be improved by patterning the opticalfilm over the phosphor layer in the color conversion layer. FIG. 53 is across-sectional side view of an illustrative color conversion layer 34showing how film 246 may be patterned to include protrusions 272. Film246 may be laminated directly to phosphor layer 40 or formed integrallywith phosphor layer 40. The presence of protrusions 272 on the uppersurface of film 246 may broaden the light that passes through colorconversion layer 34. The protrusions may spread the blue light from thelight-emitting diodes (e.g., by recycling more on-axis blue light thuscreating a broader profile for the blue light). Protrusions 272 may bepyramidal shaped protrusions, cone shaped protrusions, hemisphericalshaped protrusions, or protrusions of any other desired shape. Film 246may sometimes be referred to as a prism film and protrusions 272 maysometimes be referred to as prisms.

FIG. 54 is a cross-sectional side view of an illustrative colorconversion layer 34 showing another embodiment for improving uniformityof the display. In FIG. 54, the upper surface of phosphor layer 40 ispatterned to have protrusions 274. The presence of protrusions 274 mayresult in on-axis light and off-axis light having a similar path lengthwithin phosphor layer 40 (similar to as shown and discussed inconnection with FIGS. 49 and 50). Protrusions 274 may be pyramidalshaped protrusions, cone shaped protrusions, hemispherical shapedprotrusions, or protrusions of any other desired shape. The lowersurface of film 246 may conform to the upper surface of phosphor layer40 (e.g., film 246 has recesses that receive the protrusions 274).

If desired, the concepts from FIGS. 53 and 54 may be combined into asingle color conversion layer. FIG. 55 shows a cross-sectional side viewof a color conversion layer of this type. As shown, film 246 may bepatterned to include protrusions 272. Film 246 may be laminated directlyto phosphor layer 40 or formed integrally with phosphor layer 40. Thepresence of protrusions 272 on the upper surface of film 246 may broadenthe light that passes through color conversion layer 34 (e.g., theprotrusions may spread the blue light from the light-emitting diodes).Protrusions 272 may be pyramidal shaped protrusions, cone shapedprotrusions, hemispherical shaped protrusions, or protrusions of anyother desired shape. The upper surface of phosphor layer 40 may also bepatterned to have protrusions such as protrusions 274 in FIG. 55. Thepresence of protrusions 274 may result in on-axis light and off-axislight having a similar path length within phosphor layer 40. Protrusions274 may be pyramidal shaped protrusions, cone shaped protrusions,hemispherical shaped protrusions, or protrusions of any other desiredshape.

FIG. 56 is a diagram showing an illustrative method for attachinglight-emitting diodes to a printed circuit board. One option forattaching light-emitting diodes to a printed circuit board is to depositsolder balls on the printed circuit board. Then, the light-emittingdiode may be placed on the solder balls and the solder may be reflowedto attach the light-emitting diode to the printed circuit board. Thismethod requires two placement steps (e.g., one for depositing the solderand one for placing the light-emitting diode). This may undesirablyincrease the possible manufacturing error and increase the time it takesto attach the light-emitting diodes to the printed circuit board.

FIG. 56 shows an alternate method for attaching light-emitting diodes tothe printed circuit board in which the light-emitting diodes arepre-soldered before being placed on the printed circuit board. As shownin FIG. 56, at step 302 an adhesive layer 282 may be deposited over theprinted circuit board. Adhesive layer 282 may be any desired type ofadhesive. Next at step 304, a pre-soldered light-emitting diode 38 thatincludes solder 284 on solder pad 286 may be attached to printed circuitboard 50. The light-emitting diode may be biased in direction 288towards adhesive 282 such that adhesive 282 secures the light-emittingdiode to the printed circuit board. After securing the light-emittingdiode to the printed circuit board using adhesive 282, reflow may beperformed at step 306. Adhesive 282 may evaporate during the reflowprocess (e.g., adhesive 282 may have a boiling point lower than themelting point of solder 284). Adhesive 282 may have a melting point thatis lower than the melting point of solder 284. During reflow, solder 284may form an electrical contact with printed circuit board 50. Thelight-emitting diode 38 is therefore attached to printed circuit board50 using solder 284. Attaching light-emitting diodes using the method ofFIG. 56 (with pre-soldered light-emitting diodes) may result in betterlight-emitting diode alignment and may be faster than attachinglight-emitting diodes by separately depositing solder.

FIGS. 57-74 show cross-sectional side views of backlight unitsincorporating the aforementioned features. As shown in FIG. 57,backlight unit 42 may include a plurality of optical films formed overlight-emitting diode array 36. Light-emitting diode array 36 may containa two-dimensional array of light sources such as light-emitting diodes38 that produce backlight illumination. Light-emitting diodes 38 may, asan example, be arranged in rows and columns and may lie in the X-Y planeof FIG. 57. Light-emitting diodes 38 may be mounted on printed circuitboard 50 (sometimes referred to as substrate 50). A reflective layer 96may be formed on an upper surface of printed circuit board 50 toincrease efficiency of the backlight unit (similar to as shown in FIG.22). The reflective layer may be formed from any desired material. Inone possible arrangement, the reflective layer may also serve as asolder mask layer during attachment of the light-emitting diodes to thesubstrate (as with reflective layer 72 in FIG. 11).

Light-emitting diodes 38 may be controlled in unison by controlcircuitry in device 10 or may be individually controlled (e.g., toimplement a local dimming scheme that helps improve the dynamic range ofimages displayed on pixel array 24). The light produced by eachlight-emitting diode 38 may travel upwardly along dimension Z throughthe optical films before passing through a pixel array.

Optical films within the backlight unit may include films such as lightspreading layer 28, partially reflective layer 30, color conversionlayer 34 (which may include phosphor layer 40 and partially reflectivelayer 41), collimating layer 44 (sometimes referred to as microlensarray 44), brightness enhancement films 46-1 and 46-2, diffuser layer48, and/or other optical films.

Light-emitting diodes 38 may emit light of any suitable color (e.g.,blue, red, green, white, etc.). With one illustrative configurationdescribed herein, light-emitting diodes 38 emit blue light. To helpprovide uniform backlight across backlight unit 42, light fromlight-emitting diodes 38 may be spread by light spreading layer 28. Asshown in FIG. 57, light spreading layer 28 includes first lightspreading features 402 on the upper surface of light spreading layer 28and light spreading features 404 on the lower surface of light spreadinglayer 28. In the example of FIG. 57, light spreading features 402(sometimes referred to as light spreading structures 402, prisms 402,lenses 402, etc.) are convex microlenses and light spreading features404 (sometimes referred to as light spreading structures 404, prisms404, lenses 404, etc.) are concave microlenses. This example is merelyillustrative and both sets of light spreading features may have anydesired shape and may be convex features (e.g., protrusions) or concavefeatures (e.g., recesses).

After passing through light spreading layer 28, light fromlight-emitting diodes 38 may pass through partially reflective layer 30.Partially reflective layer 30 (sometimes referred to as dichroic layer30 or dichroic filter layer 30) may be configured to reflect some lightfrom the LEDs and transmit some light from the LEDs. As shown in FIG.57, partially reflective layer 30 may include a multi-Bragg reflector406 and a diffuser layer 408 that are laminated together. Light that isreflected off multi-Bragg reflector 406 may be recycled (e.g., thereflected light will reflect off of other layers such as substrate 50before reaching multi-Bragg reflector 406 again). Light that istransmitted through multi-Bragg reflector 406 then passes through colorconversion layer 34 (which may sometimes be referred to as aphotoluminescent layer).

The transmission of multi-Bragg reflector 406 may be selected tomaximize the efficiency of display 14. Lowering the transmission of bluelight (e.g., from the light-emitting diodes) through the partiallyreflective layer increases the amount of blue light that is recycled.However, recycling more light may cause more light to be absorbed byprinted circuit board 50 (or other layers below multi-Bragg reflector406). Increasing the transmission of blue light may cause more visibleartifacts. Therefore, the transmission of the partially reflective layermay be selected to optimize efficiency and uniformity of the display.The reflectance of printed circuit board 50 may influence the optimumtransmission level of partially reflective layer 30. In one illustrativeembodiment, printed circuit board 50 may have a reflectance of about 90%and multi-Bragg reflector 406 may reflect 50% of blue light fromlight-emitting diodes 38. Increasing the reflectance of printed circuitboard 50 increases the optimum reflectance of partially reflective layer30.

Color conversion layer 34 may convert the light from LEDs 38 from afirst color to a different color. For example, when the LEDs emit bluelight, color conversion layer 34 may include a phosphor layer 40 (e.g.,a layer of white phosphor material or other photoluminescent material)that converts blue light into white light. If desired, otherphotoluminescent materials may be used to convert blue light to light ofdifferent colors (e.g., red light, green light, white light, etc.). Forexample, one layer 34 may have a phosphor layer 40 that includes quantumdots that convert blue light into red and green light (e.g., to producewhite backlight illumination that includes, red, green, and bluecomponents, etc.). Configurations in which light-emitting diodes 38 emitwhite light (e.g., so that layer 34 may be omitted, if desired) may alsobe used. In addition to phosphor layer 40, color conversion layer 34 mayinclude a partially reflective layer 41. Partially reflective layer 41(sometimes referred to as a dichroic layer or dichroic filter layer) mayreflect all red and green light and partially reflect blue light, forexample. Phosphor layer 40 and partially reflective layer 41 may belaminated together to form a single integral color conversion layer 34.

Collimating layer 44 (sometimes referred to as microlens layer 44 ormicrolens array diffuser 44) may collimate off-axis light. As shown inFIG. 57, collimating layer 44 may include an array of microlenses 162 onthe lower surface of the film. Backlight unit 42 also includesbrightness enhancement films such as brightness enhancement film 46-1and brightness enhancement film 46-2 to further collimate the light. Asshown, brightness enhancement film 46-1 may include protrusions 160 onan upper surface of the film. An additional optical layer 410 may beinterposed between brightness enhancement film 46-1 and collimatinglayer 44. The additional optical layer 410 may be formed from atransparent material that has a low index-of-refraction. Optical layer410 may therefore sometimes be referred to as low-index layer 410. Asshown in FIG. 57, brightness enhancement film 46-1, low-index layer 410,and collimating layer 44 may be laminated together to form a combinedoptical layer 412.

An additional brightness enhancement film 46-2 may be included overbrightness enhancement film 46-1 to further help collimate the backlightand thereby increase the brightness of the display. Brightnessenhancement films 46-1 and 46-2 may optionally have protrusions thatextend along longitudinal axes that are rotated 90° relative to eachother if desired. Finally, backlight unit 42 may include diffuser layer48 to homogenize light from the array of light-emitting diodes.

The example of FIG. 57 of light spreading layer 28 including convexlight spreading features on an upper surface and concave light spreadingfeatures on a lower surface is merely illustrative. In general, bothsets of light spreading features may have any desired shape and may beconvex or concave. FIG. 58 is a cross-sectional side view of anillustrative backlight unit with concave light spreading features 402 onan upper surface and concave light spreading features 404 on a lowersurface.

Also in FIG. 58, collimating layer 44 includes microlenses 163 on theupper surface of the film in addition to microlenses 162 on the lowersurface of the film. Microlenses 162 and 163 (sometimes referred to asrecesses 162 and 163 or surface features 162 and 163) may have anydesired shape. Collimating layer 44 and brightness enhancement film 46-1are formed as separate films in FIG. 58 (instead of being laminatedtogether with a low-index layer as in FIG. 57). In general, in all ofthe embodiments herein collimating layer 44 and brightness enhancementfilm 46-1 may be formed separately or may be laminated together.

Additional layers within the backlight unit may also be laminatedtogether if desired. FIG. 59 is a cross-sectional side view of abacklight unit similar to the backlight unit of FIG. 57. However, inFIG. 59 the phosphor layer 40, partially reflective layer 41,multi-Bragg reflector 406, and diffuser layer 408 may be laminatedtogether to form an integral conversion film 414.

FIG. 60 is a cross-sectional side view of an illustrative backlight unitwith first and second light spreading layers. As shown, light spreadinglayer 28-1 is positioned above light spreading layer 28-2. Lightspreading layer 28-1 and light spreading layer 28-2 may be attachedusing adhesive. Alternatively, light spreading layers 28-1 and 28-2 maybe laminated together to form an integral film. As shown in FIG. 60,light spreading layer 28-2 has an upper surface with microlenses 132.Microlenses 132 may be formed from a plurality of recesses in the uppersurface of light spreading layer 28-2. Light spreading layer 28-2 mayalso include a plurality of protrusions 130. Protrusions 130 mayprotrude towards the light-emitting diodes. Protrusions 130 may beelongated protrusions (sometimes referred to as ridges) that extendalong a longitudinal axis across the layer (e.g., parallel to the Y-axisin FIG. 60). Light spreading layer 28-1 may be the same as lightspreading layer 28-2, except for being rotated 90° relative to lightspreading layer 28-2. Light spreading layer 28-1 also includesmicrolenses 126. Microlenses 126 may be formed from a plurality ofrecesses in the upper surface of light spreading layer 28-1. Microlenses126 and 132 may optionally be protrusions instead of recesses ifdesired.

Light emitted from light-emitting diodes 38 will be spread relative to afirst axis (e.g., from a point source to two points) upon reachinglight-spreading layer 28-2. Microlenses 132 may reduce total internalreflection to promote light passing to light spreading layer 28-1. Lightspreading layer 28-1 will spread the incoming light relative to a secondaxis (e.g., from the two points to four points) that is perpendicular tothe first axis (because the protrusions of layer 28-1 are perpendicularto the protrusions of layer 28-2). Microlenses 126 may reduce totalinternal reflection to promote leakage of light out of layer 28-1towards layer 30. Layers 28-1 and 28-2 may each have any desiredthickness (e.g., between 20 and 30 microns, between 20 and 25 microns,less than 50 microns, less than 25 microns, about 22 microns, greaterthan 15 microns, less than 100 microns, etc.). The thickness of layers28-1 and 28-2 may be the same.

FIG. 61 is a cross-sectional side view of a backlight having protrusions272 formed over phosphor layer 40 in color conversion layer 34. Thepresence of protrusions 272 may broaden the light that passes throughcolor conversion layer 34 (e.g., the protrusions may spread the bluelight from the light-emitting diodes). Protrusions 272 may be pyramidalshaped protrusions, cone shaped protrusions, hemispherical shapedprotrusions, or protrusions of any other desired shape. Protrusions 272may sometimes be referred to as prisms and may be formed in a film(prism film). Instead of protrusions, an optical film laminated tophosphor layer 40 may include recesses having any desired shape. Due tothe presence of protrusions 272, collimating layer 44 may optionally beomitted from the backlight as shown in FIG. 61. Collimating layer 44 mayoptionally be omitted from any of the embodiments herein.

Any of the color conversion layers shown in FIGS. 53-55 may be used inthe backlight units described herein. The example of FIG. 61 of thecolor conversion layer having protrusions 272 (similar to FIG. 53) ismerely illustrative. The color conversion layer may instead have apatterned phosphor layer (as in FIG. 54) or may have both a patternedphosphor layer and patterned film over the phosphor layer (as in FIG.55). The features of the patterned phosphor layer and patterned filmover the phosphor layer may have any desired shapes.

The example in FIG. 61 of light spreading layer 28 including two layers(28-1 and 28-2) having elongated protrusions that extend along alongitudinal axis is merely illustrative. If desired, light spreadinglayer 28 may instead include an array of protrusions or recesses, asshown in FIG. 62. As shown in FIG. 62, light spreading layer 28 includeslight spreading features 422 on an upper surface and light spreadingfeatures 424 on a lower surface. Light spreading features 422 and 424may have any desired shape. For example, the light spreading features(sometimes referred to as light spreading structures 422 or lightspreading structures 424) may be protrusions having a pyramidal shape(e.g., with a square base and four triangular faces that meet at avertex) or a triangular pyramidal shape (e.g., with a triangular baseand three triangular faces that meet at a vertex). The pyramidalprotrusions may split a point light source into four points, whereas thetriangular pyramidal protrusions may split a point light source intothree points. In another embodiment, the light spreading structures maybe recesses having a pyramidal shape or a triangular pyramidal shape.Any of the light spreading structures of FIGS. 33-35 may be included inthe light spreading layer. The shapes of the structures on the uppersurface of layer 28 may be the same or may be different than the shapesof the structures on the lower surface of layer 28. Layer 28 may beformed from a single optical film with light spreading structurespatterned on the upper and lower surface. Layer 28 may also be formedfrom first and second optical films that are laminated together and thathave respective light spreading structures.

FIG. 62 also shows an example of an encapsulant arrangement for thelight-emitting diodes 38 on printed circuit board 50. As shown in insetportion 426, light-emitting diode 38 is mounted to printed circuit board50. Reflective layer 96 is formed on printed circuit board 50 around thelight-emitting diode. Each light-emitting diode 38 may include areflector layer 70 (e.g., a distributed Bragg-reflector) formed over thelight-emitting diode. The light-emitting diode is also covered byencapsulant such as dome-shaped encapsulant 52. Reflector layer 70 andreflective layer 96 may help spread the light from light-emitting diode38 to avoid hotspots.

The arrangement for the encapsulant and light-emitting diode in FIG. 62is merely illustrative. FIG. 63 is cross-sectional side view of anotherbacklight with a different possible LED arrangement depicted in insetportion 426. As shown in FIG. 63, structures 428 (sometimes referred toas support structures, reflective structures, or light leakage promotionstructures) may be formed on substrate 50. Structures 428 may betransparent structures configured to provide mechanical support for thebacklight unit (similar to structures 232 in FIG. 47). Structures 428may alternatively be reflective structures configured to redirect lightfrom light-emitting diode 38 towards the user (as shown in FIG. 63).

In FIG. 63, structures 428 are depicted as having a triangularcross-sectional shape. However, this example is merely illustrative andstructures 428 may have any desired shape. Structures 428 may have atrapezoidal cross-sectional shape or a cross-sectional shape with acurved upper surface. FIG. 64 shows an example where structures 428 havea curved upper surface.

FIG. 65 shows yet another possible arrangement for the light-emittingdiodes on printed circuit board 50 in inset portion 426. As shown inFIG. 65, encapsulant 52 may be formed as a slab between structures 428(as opposed to a dome of encapsulant as in FIG. 64). The encapsulant mayhave a planar upper surface or an upper surface with a slight convexcurve.

Inset portion 426 of FIG. 66 shows another arrangement forlight-emitting diodes 38 and encapsulant 52. In FIG. 66, structures 428(from FIG. 65) are omitted and encapsulant 52 is formed as a slab overthe light-emitting diodes. The encapsulant may have a planar uppersurface and may have a thickness that is tuned for optimal spreading oflight from the light-emitting diodes.

In another embodiment, shown in inset portion 426 of FIG. 67, additionalreflective material 430 may be incorporated on the slab of encapsulant52. Additional reflective material 430 may be translucent and may causespecular reflection (in which the angle of incidence of a ray of lightequals the angle of output of the reflected ray of light) ad/ordiffusive reflection (in which an incident ray of light may be reflectedin any direction). Reflective material 430 may be patterned to haveportions formed directly over corresponding light-emitting diodes. Thereflective material may be printed directly on encapsulant 52 or may beformed on a separate film that is aligned over the light-emittingdiodes. Reflective material 430 may reflect at least 10% of light, atleast 25% of light, at least 50% of light, at least 75% of light, lessthan 90% of light, or any other desired amount of light.

Additional reflective material 430 may be included in embodiments whereadditional structures 428 are present. FIG. 68 shows an embodiment ofthis type. As shown, encapsulant 52 is formed between reflectivestructures 428 and reflective material 430 is formed on the encapsulant.Reflective material 430 may reflect at least 10% of light, at least 25%of light, at least 50% of light, at least 75% of light, less than 90% oflight, or any other desired amount of light.

In FIGS. 62-68, reflective layer 96 is depicted on the upper surface ofprinted circuit board 50. Reflective layer 96 may cause diffusivereflection, meaning an incident ray of light may be reflected in anydirection. This helps direct light towards the viewer in areas betweenthe light-emitting diodes. However, in some embodiments, additionalreflective materials may be included that cause specular reflection.FIG. 69 is a cross-sectional side view of a backlight unit that includesreflective material 446 formed in a ring around light-emitting diode 70.Reflective material 446 may cause specular reflection, meaning anincident ray of light will be reflected at the same angle it strikes thereflective material. Reflective material 446 (sometimes referred to asspecular reflective material 446) may help spread light further awayfrom light-emitting diode 38 before the light is directed towards theviewer by reflective material 96 (sometimes referred to as diffusivereflective material 96).

FIGS. 62-69 show various arrangements for light-emitting diode 38 andencapsulant 52 on substrate 50. It should be understood that for all ofthe embodiments herein (e.g. in FIGS. 57-61 and FIGS. 70-73), any of thearrangements of FIGS. 62-69 may optionally be used or no encapsulant maybe included.

FIGS. 70 and 71 are cross-sectional side views of an illustrativebacklight unit showing how the design of light-emitting diodes 38,encapsulant 52, and structures 428 may allow partially reflective layer30 to be omitted from the backlight unit. Omitting the partiallyreflective layer 30 may allow for the thickness of the backlight unit tobe reduced. FIG. 70 shows an example where light spreading layer 28includes light spreading features on the upper and lower surface and thepartially reflective layer is omitted. FIG. 71 shows an example wherelight spreading layer 28 includes layers 28-1 and 28-2 rotated 90°relative to each other (as in FIG. 60) and the partially reflectivelayer is omitted. Layers 28-1 and 28-2 in FIG. 71 are laminatedtogether.

It should be noted that, in any of the embodiments described herein,partially reflective layer 41 may optionally be omitted. The lightspreading provided by the patterned film on phosphor layer 40 withprotrusions 272 may provide sufficient light spreading such that thereflective properties of partially reflective layer 41 are not requiredfor satisfactory display performance. In this case, partially reflectivelayer 41 may be replaced by a transparent optical layer withoutpartially reflective properties. The transparent optical layer may stillprovide mechanical support for phosphor layer 40.

FIG. 72 is a cross-sectional side view of an illustrative backlight unitwith a color conversion layer that includes reflective structures. Asshown in FIG. 72, reflective structures 256 may be formed in phosphorlayer. In other words, the color conversion layer of FIG. 50A may beused. Other color conversion layers with reflective structures may beused. For example, color conversion layers with reflective structuresformed in a film over the phosphor layer (as in FIG. 50B or 50C) mayalso be used if desired. The reflective structures may have any desiredarrangement when viewed from above. FIGS. 51A and 51B show illustrativearrangements for the reflective structures that may be used in FIG. 72.Structures 256 may optionally be cylinders, cones, pyramids, or othershapes. The reflective structures may be arranged in an interconnectedweb that defines a plurality of triangular openings, a plurality ofcircular openings, a plurality of square openings, a plurality ofhexagonal openings, a plurality of octagonal openings, a plurality ofnon-square rectangular openings, etc. In one embodiment, the reflectivestructures (sometimes referred to as reflective walls) may have openingsto allow the phosphor material to flow between cavities.

FIG. 73 is a cross-sectional side view of an illustrative backlightsimilar to the backlight of FIG. 72. However, in FIG. 73 light-emittingdiodes 38 have been dithered, as shown by inset portion 442. Thelight-emitting diodes may be dithered similar to as shown in connectionwith FIGS. 43-45. Any of the embodiments herein may optionally includedithered light-emitting diodes of the type shown in FIG. 73.

The foregoing is merely illustrative and various modifications can bemade to the described embodiments. The foregoing embodiments may beimplemented individually or in any combination.

What is claimed is:
 1. A display, comprising: a plurality of pixels; anda backlight configured to produce backlight illumination for theplurality of pixels, wherein the backlight comprises: light sources thatare configured to emit light and that are arranged in a plurality ofrespective cells; a first light spreading layer that receives light fromthe light sources and has a first plurality of elongated protrusionsthat extend in a first direction; and a second light spreading layerthat receives light from the first light spreading layer and has asecond plurality of elongated protrusions that extend in a seconddirection that is perpendicular to the first direction, wherein thefirst light spreading layer has a first array of microlenses formed onan upper surface of the first light spreading layer and wherein thesecond light spreading layer has a second array of microlenses formed onan upper surface of the second light spreading layer.
 2. The displaydefined in claim 1, wherein the first plurality of elongated protrusionsextend away from a lower surface of the first light spreading layer andwherein the second plurality of elongated protrusions extend away from alower surface of the second light spreading layer.
 3. The displaydefined in claim 2, wherein each protrusion of the first plurality ofelongated protrusions has a triangular cross-section and wherein eachprotrusion of the second plurality of elongated protrusions has atriangular cross-section.
 4. The display defined in claim 3, wherein thefirst array of microlenses are formed from a first plurality of recesseson the upper surface of the first light spreading layer and wherein thesecond array of microlenses are formed from a second plurality ofrecesses on the upper surface of the second light spreading layer. 5.The display defined in claim 1, wherein the light sources compriselight-emitting diodes mounted on a printed circuit board.
 6. The displaydefined in claim 5, wherein each light-emitting diode has first andsecond opposing sides, wherein the first side of each light-emittingdiode is attached to the printed circuit board, and wherein a reflectorlayer is formed on the second side of each light-emitting diode.
 7. Thedisplay defined in claim 5, wherein the backlight further comprisesfirst and second prisms mounted on the printed circuit board betweeneach pair of adjacent light-emitting diodes.
 8. The display defined inclaim 1, wherein the first and second light spreading layers areattached by a layer of adhesive.
 9. The display defined in claim 1,wherein the first light spreading layer is laminated to the second lightspreading layer.
 10. A display, comprising: a plurality of pixels; and abacklight configured to produce backlight illumination for the pluralityof pixels, wherein the backlight comprises: light sources that areconfigured to emit light and that are arranged in a plurality ofrespective cells, wherein the light sources comprise light-emittingdiodes mounted on an upper surface of a printed circuit board; a firstlight spreading layer that receives light from the light sources and hasa first plurality of elongated protrusions that extend in a firstdirection; a second light spreading layer that receives light from thefirst light spreading layer and has a second plurality of elongatedprotrusions that extend in a second direction that is perpendicular tothe first direction; and a reflective layer formed on the upper surfaceof the printed circuit board.
 11. The display defined in claim 10,wherein the reflective layer comprises a diffusive reflective material.12. A display, comprising: a plurality of pixels; and a backlightconfigured to produce backlight illumination for the plurality ofpixels, wherein the backlight comprises: light sources that areconfigured to emit light and that are arranged in a plurality ofrespective cells, wherein the light sources comprise light-emittingdiodes mounted on a printed circuit board; a first light spreading layerthat receives light from the light sources and has a first plurality ofelongated protrusions that extend in a first direction; a second lightspreading layer that receives light from the first light spreading layerand has a second plurality of elongated protrusions that extend in asecond direction that is perpendicular to the first direction; and alight leakage promotion structure that is mounted on the printed circuitboard and that is interposed between adjacent light-emitting diodes. 13.A display, comprising: a plurality of pixels; and a backlight configuredto produce backlight illumination for the plurality of pixels, whereinthe backlight comprises: a printed circuit board; a plurality oflight-emitting diodes mounted on the printed circuit board; at least onelight spreading layer formed over the printed circuit board that spreadslight received from the plurality of light-emitting diodes; and aplurality of reflective structures mounted on the printed circuit board,wherein each reflective structure is interposed between a respectivepair of adjacent light-emitting diodes.
 14. The display defined in claim13, wherein the at least one light spreading layer comprises: a firstlight spreading layer with an upper surface having protrusions; and apolarization grating layer that is formed on a lower surface of thefirst light spreading layer.
 15. The display defined in claim 13,wherein the at least one light spreading layer comprises a first lightspreading layer having unevenly spaced protrusions on an upper surface,a polarization grating layer, and a wave guide layer interposed betweenthe polarization grating layer and the first light spreading layer. 16.The display defined in claim 13, wherein the at least one lightspreading layer comprises a first light spreading layer with an array ofpyramidal protrusions.
 17. The display defined in claim 13, wherein theat least one light spreading layer comprises a first light spreadinglayer with an array of protrusions and wherein each protrusion has atapered pyramid shape.
 18. The display defined in claim 13, wherein theat least one light spreading layer comprises: a first light spreadinglayer that receives light from the plurality of light-emitting diodesand has a first plurality of elongated protrusions that extend in afirst direction; and a second light spreading layer that receives lightfrom the first light spreading layer and has a second plurality ofelongated protrusions that extend in a second direction that isperpendicular to the first direction.
 19. A display, comprising: aplurality of pixels; and a backlight configured to produce backlightillumination for the plurality of pixels, wherein the backlightcomprises: a printed circuit board; a plurality of light-emitting diodesmounted on the printed circuit board; a light spreading layer formedover the printed circuit board that spreads light received from theplurality of light-emitting diodes, wherein the light spreading layerhas an upper surface and a lower surface, wherein a first plurality oflight spreading features are formed on the upper surface, and wherein asecond plurality of light spreading features are formed on the lowersurface; and a color conversion layer formed over the light spreadinglayer.